diff --git a/compiler/fir/dump/src/org/jetbrains/kotlin/fir/dump/HtmlFirDump.kt b/compiler/fir/dump/src/org/jetbrains/kotlin/fir/dump/HtmlFirDump.kt
index e75a2220a4d..32f50c83d90 100644
--- a/compiler/fir/dump/src/org/jetbrains/kotlin/fir/dump/HtmlFirDump.kt
+++ b/compiler/fir/dump/src/org/jetbrains/kotlin/fir/dump/HtmlFirDump.kt
@@ -758,7 +758,7 @@ class HtmlFirDump internal constructor(private var linkResolver: FirLinkResolver
 
     private fun FlowContent.generate(typeRef: FirTypeRef) {
         when (typeRef) {
-            is FirErrorTypeRef -> error { +typeRef.reason }
+            is FirErrorTypeRef -> error { +typeRef.diagnostic.reason }
             is FirResolvedTypeRef -> generate(typeRef.type)
             is FirImplicitTypeRef -> unresolved { keyword("<implicit>") }
             is FirUserTypeRef -> unresolved {
@@ -956,7 +956,7 @@ class HtmlFirDump internal constructor(private var linkResolver: FirLinkResolver
             }
             is FirErrorNamedReference -> {
                 error {
-                    title = reference.errorReason
+                    title = reference.diagnostic.reason
                     simpleName(reference.name)
                 }
             }
diff --git a/compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrVisitor.kt b/compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrVisitor.kt
index c7840b59403..065463f950b 100644
--- a/compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrVisitor.kt
+++ b/compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrVisitor.kt
@@ -969,7 +969,7 @@ class Fir2IrVisitor(
             IrErrorExpressionImpl(
                 startOffset, endOffset,
                 errorExpression.typeRef.toIrType(session, declarationStorage),
-                errorExpression.reason
+                errorExpression.diagnostic.reason
             )
         }
     }
diff --git a/compiler/fir/java/src/org/jetbrains/kotlin/fir/java/JavaUtils.kt b/compiler/fir/java/src/org/jetbrains/kotlin/fir/java/JavaUtils.kt
index 2b843f794e9..484c947ca1c 100644
--- a/compiler/fir/java/src/org/jetbrains/kotlin/fir/java/JavaUtils.kt
+++ b/compiler/fir/java/src/org/jetbrains/kotlin/fir/java/JavaUtils.kt
@@ -12,6 +12,7 @@ import org.jetbrains.kotlin.fir.FirSession
 import org.jetbrains.kotlin.fir.declarations.FirFunction
 import org.jetbrains.kotlin.fir.declarations.FirTypeParameter
 import org.jetbrains.kotlin.fir.declarations.FirValueParameter
+import org.jetbrains.kotlin.fir.diagnostics.FirSimpleDiagnostic
 import org.jetbrains.kotlin.fir.expressions.FirAnnotationCall
 import org.jetbrains.kotlin.fir.expressions.FirArrayOfCall
 import org.jetbrains.kotlin.fir.expressions.FirExpression
@@ -260,7 +261,7 @@ private fun JavaAnnotationArgument.toFirExpression(
                     null
                 }
                 this.calleeReference = calleeReference
-                    ?: FirErrorNamedReferenceImpl(null, "Strange Java enum value: $classId.$entryName")
+                    ?: FirErrorNamedReferenceImpl(null, FirSimpleDiagnostic("Strange Java enum value: $classId.$entryName"))
             }
         }
         is JavaClassObjectAnnotationArgument -> FirGetClassCallImpl(null).apply {
@@ -270,7 +271,7 @@ private fun JavaAnnotationArgument.toFirExpression(
             )
         }
         is JavaAnnotationAsAnnotationArgument -> getAnnotation().toFirAnnotationCall(session, javaTypeParameterStack)
-        else -> FirErrorExpressionImpl(null, "Unknown JavaAnnotationArgument: ${this::class.java}")
+        else -> FirErrorExpressionImpl(null, FirSimpleDiagnostic("Unknown JavaAnnotationArgument: ${this::class.java}"))
     }
 }
 
@@ -304,7 +305,7 @@ internal fun Any?.createConstant(session: FirSession): FirExpression {
         is BooleanArray -> toList().createArrayOfCall(session, IrConstKind.Boolean)
         null -> FirConstExpressionImpl(null, IrConstKind.Null, null)
 
-        else -> FirErrorExpressionImpl(null, "Unknown value in JavaLiteralAnnotationArgument: $this")
+        else -> FirErrorExpressionImpl(null, FirSimpleDiagnostic("Unknown value in JavaLiteralAnnotationArgument: $this"))
     }
 }
 
diff --git a/compiler/fir/java/src/org/jetbrains/kotlin/fir/java/deserialization/KotlinDeserializedJvmSymbolsProvider.kt b/compiler/fir/java/src/org/jetbrains/kotlin/fir/java/deserialization/KotlinDeserializedJvmSymbolsProvider.kt
index 424127f3386..83b53eca11f 100644
--- a/compiler/fir/java/src/org/jetbrains/kotlin/fir/java/deserialization/KotlinDeserializedJvmSymbolsProvider.kt
+++ b/compiler/fir/java/src/org/jetbrains/kotlin/fir/java/deserialization/KotlinDeserializedJvmSymbolsProvider.kt
@@ -14,6 +14,7 @@ import org.jetbrains.kotlin.fir.declarations.impl.FirSimpleFunctionImpl
 import org.jetbrains.kotlin.fir.declarations.impl.FirPropertyImpl
 import org.jetbrains.kotlin.fir.deserialization.FirDeserializationContext
 import org.jetbrains.kotlin.fir.deserialization.deserializeClassToSymbol
+import org.jetbrains.kotlin.fir.diagnostics.FirSimpleDiagnostic
 import org.jetbrains.kotlin.fir.expressions.FirAnnotationCall
 import org.jetbrains.kotlin.fir.expressions.FirClassReferenceExpression
 import org.jetbrains.kotlin.fir.expressions.FirExpression
@@ -174,7 +175,7 @@ class KotlinDeserializedJvmSymbolsProvider(
             FirResolvedTypeRefImpl(
                 null, it.constructType(emptyList(), isNullable = false)
             )
-        } ?: FirErrorTypeRefImpl(null, "Symbol not found for $classId")
+        } ?: FirErrorTypeRefImpl(null, FirSimpleDiagnostic("Symbol not found for $classId"))
 
     }
 
@@ -226,7 +227,7 @@ class KotlinDeserializedJvmSymbolsProvider(
                         else -> {
                             FirErrorNamedReferenceImpl(
                                 null,
-                                errorReason = "Strange deserialized enum value: ${this@toEnumEntryReferenceExpression}.$name"
+                                FirSimpleDiagnostic("Strange deserialized enum value: ${this@toEnumEntryReferenceExpression}.$name")
                             )
                         }
                     }
diff --git a/compiler/fir/lightTree/src/org/jetbrains/kotlin/fir/lightTree/converter/DeclarationsConverter.kt b/compiler/fir/lightTree/src/org/jetbrains/kotlin/fir/lightTree/converter/DeclarationsConverter.kt
index ae0d94c3a5c..468cb7ad561 100644
--- a/compiler/fir/lightTree/src/org/jetbrains/kotlin/fir/lightTree/converter/DeclarationsConverter.kt
+++ b/compiler/fir/lightTree/src/org/jetbrains/kotlin/fir/lightTree/converter/DeclarationsConverter.kt
@@ -22,6 +22,7 @@ import org.jetbrains.kotlin.fir.builder.generateComponentFunctions
 import org.jetbrains.kotlin.fir.builder.generateCopyFunction
 import org.jetbrains.kotlin.fir.declarations.*
 import org.jetbrains.kotlin.fir.declarations.impl.*
+import org.jetbrains.kotlin.fir.diagnostics.FirSimpleDiagnostic
 import org.jetbrains.kotlin.fir.expressions.*
 import org.jetbrains.kotlin.fir.expressions.impl.*
 import org.jetbrains.kotlin.fir.lightTree.LightTree2Fir
@@ -664,7 +665,7 @@ class DeclarationsConverter(
         }
 
         val delegatedSelfTypeRef =
-            if (classWrapper.isObjectLiteral()) FirErrorTypeRefImpl(null, "Constructor in object")
+            if (classWrapper.isObjectLiteral()) FirErrorTypeRefImpl(null, FirSimpleDiagnostic("Constructor in object"))
             else classWrapper.delegatedSelfTypeRef
 
         val status = FirDeclarationStatusImpl(modifiers.getVisibility(), Modality.FINAL).apply {
@@ -714,8 +715,8 @@ class DeclarationsConverter(
         val isThis = (isImplicit && classWrapper.hasPrimaryConstructor) || thisKeywordPresent
         val delegatedType =
             if (classWrapper.isObjectLiteral() || classWrapper.isInterface()) when {
-                isThis -> FirErrorTypeRefImpl(null, "Constructor in object")
-                else -> FirErrorTypeRefImpl(null, "No super type")
+                isThis -> FirErrorTypeRefImpl(null, FirSimpleDiagnostic("Constructor in object"))
+                else -> FirErrorTypeRefImpl(null, FirSimpleDiagnostic("No super type"))
             }
             else when {
                 isThis -> classWrapper.delegatedSelfTypeRef
@@ -869,7 +870,7 @@ class DeclarationsConverter(
     private fun convertDestructingDeclaration(destructingDeclaration: LighterASTNode): DestructuringDeclaration {
         var isVar = false
         val entries = mutableListOf<FirVariable<*>>()
-        var firExpression: FirExpression = FirErrorExpressionImpl(null, "Destructuring declaration without initializer")
+        var firExpression: FirExpression = FirErrorExpressionImpl(null, FirSimpleDiagnostic("Destructuring declaration without initializer"))
         destructingDeclaration.forEachChildren {
             when (it.tokenType) {
                 VAR_KEYWORD -> isVar = true
@@ -1165,7 +1166,7 @@ class DeclarationsConverter(
      */
     private fun convertExplicitDelegation(explicitDelegation: LighterASTNode): FirDelegatedTypeRef {
         lateinit var firTypeRef: FirTypeRef
-        var firExpression: FirExpression? = FirErrorExpressionImpl(null, "Should have delegate")
+        var firExpression: FirExpression? = FirErrorExpressionImpl(null, FirSimpleDiagnostic("Should have delegate"))
         explicitDelegation.forEachChildren {
             when (it.tokenType) {
                 TYPE_REFERENCE -> firTypeRef = convertType(it)
@@ -1255,10 +1256,10 @@ class DeclarationsConverter(
      */
     fun convertType(type: LighterASTNode): FirTypeRef {
         if (type.asText.isEmpty()) {
-            return FirErrorTypeRefImpl(null, "Unwrapped type is null")
+            return FirErrorTypeRefImpl(null, FirSimpleDiagnostic("Unwrapped type is null"))
         }
         var typeModifiers = TypeModifier() //TODO what with suspend?
-        var firType: FirTypeRef = FirErrorTypeRefImpl(null, "Incomplete code")
+        var firType: FirTypeRef = FirErrorTypeRefImpl(null, FirSimpleDiagnostic("Incomplete code"))
         var afterLPar = false
         type.forEachChildren {
             when (it.tokenType) {
@@ -1269,7 +1270,7 @@ class DeclarationsConverter(
                 NULLABLE_TYPE -> firType = convertNullableType(it)
                 FUNCTION_TYPE -> firType = convertFunctionType(it)
                 DYNAMIC_TYPE -> firType = FirDynamicTypeRefImpl(null, false)
-                TokenType.ERROR_ELEMENT -> firType = FirErrorTypeRefImpl(null, "Unwrapped type is null")
+                TokenType.ERROR_ELEMENT -> firType = FirErrorTypeRefImpl(null, FirSimpleDiagnostic("Unwrapped type is null"))
             }
         }
 
@@ -1324,7 +1325,7 @@ class DeclarationsConverter(
         }
 
         if (identifier == null)
-            return FirErrorTypeRefImpl(null, "Incomplete user type")
+            return FirErrorTypeRefImpl(null, FirSimpleDiagnostic("Incomplete user type"))
 
         val qualifier = FirQualifierPartImpl(
             identifier.nameAsSafeName()
diff --git a/compiler/fir/lightTree/src/org/jetbrains/kotlin/fir/lightTree/converter/ExpressionsConverter.kt b/compiler/fir/lightTree/src/org/jetbrains/kotlin/fir/lightTree/converter/ExpressionsConverter.kt
index 37db15c0c10..a7d7199df43 100644
--- a/compiler/fir/lightTree/src/org/jetbrains/kotlin/fir/lightTree/converter/ExpressionsConverter.kt
+++ b/compiler/fir/lightTree/src/org/jetbrains/kotlin/fir/lightTree/converter/ExpressionsConverter.kt
@@ -20,6 +20,7 @@ import org.jetbrains.kotlin.fir.declarations.impl.FirAnonymousFunctionImpl
 import org.jetbrains.kotlin.fir.declarations.impl.FirDeclarationStatusImpl
 import org.jetbrains.kotlin.fir.declarations.impl.FirPropertyImpl
 import org.jetbrains.kotlin.fir.declarations.impl.FirValueParameterImpl
+import org.jetbrains.kotlin.fir.diagnostics.FirSimpleDiagnostic
 import org.jetbrains.kotlin.fir.expressions.*
 import org.jetbrains.kotlin.fir.expressions.impl.*
 import org.jetbrains.kotlin.fir.impl.FirAbstractAnnotatedElement
@@ -56,7 +57,7 @@ class ExpressionsConverter(
 ) : BaseConverter(session, tree, context) {
 
     inline fun <reified R : FirElement> getAsFirExpression(expression: LighterASTNode?, errorReason: String = ""): R {
-        return expression?.let { convertExpression(it, errorReason) } as? R ?: (FirErrorExpressionImpl(null, errorReason) as R)
+        return expression?.let { convertExpression(it, errorReason) } as? R ?: (FirErrorExpressionImpl(null, FirSimpleDiagnostic(errorReason)) as R)
     }
 
     /*****    EXPRESSIONS    *****/
@@ -104,7 +105,7 @@ class ExpressionsConverter(
 
                 OBJECT_LITERAL -> declarationsConverter.convertObjectLiteral(expression)
                 FUN -> declarationsConverter.convertFunctionDeclaration(expression)
-                else -> FirErrorExpressionImpl(null, errorReason)
+                else -> FirErrorExpressionImpl(null, FirSimpleDiagnostic(errorReason))
             }
         }
 
@@ -158,7 +159,7 @@ class ExpressionsConverter(
                 FirLabelImpl(null, it.asString())
             }
             val bodyExpression = block?.let { declarationsConverter.convertBlockExpression(it) }
-                ?: FirErrorExpressionImpl(null, "Lambda has no body")
+                ?: FirErrorExpressionImpl(null, FirSimpleDiagnostic("Lambda has no body"))
             body = if (bodyExpression is FirBlockImpl) {
                 if (bodyExpression.statements.isEmpty()) {
                     bodyExpression.statements.add(FirUnitExpression(null))
@@ -185,7 +186,7 @@ class ExpressionsConverter(
         var isLeftArgument = true
         lateinit var operationTokenName: String
         var leftArgNode: LighterASTNode? = null
-        var rightArgAsFir: FirExpression = FirErrorExpressionImpl(null, "No right operand")
+        var rightArgAsFir: FirExpression = FirErrorExpressionImpl(null, FirSimpleDiagnostic("No right operand"))
         binaryExpression.forEachChildren {
             when (it.tokenType) {
                 OPERATION_REFERENCE -> {
@@ -249,7 +250,7 @@ class ExpressionsConverter(
         toFirOperation: String.() -> FirOperation
     ): FirTypeOperatorCall {
         lateinit var operationTokenName: String
-        var leftArgAsFir: FirExpression = FirErrorExpressionImpl(null, "No left operand")
+        var leftArgAsFir: FirExpression = FirErrorExpressionImpl(null, FirSimpleDiagnostic("No left operand"))
         lateinit var firType: FirTypeRef
         binaryExpression.forEachChildren {
             when (it.tokenType) {
@@ -285,7 +286,7 @@ class ExpressionsConverter(
             context.firLabels.removeLast()
             //println("Unused label: ${labeledExpression.getAsString()}")
         }
-        return firExpression ?: FirErrorExpressionImpl(null, "Empty label")
+        return firExpression ?: FirErrorExpressionImpl(null, FirSimpleDiagnostic("Empty label"))
     }
 
     /**
@@ -347,7 +348,7 @@ class ExpressionsConverter(
 
         return (firExpression as? FirAbstractAnnotatedElement)?.apply {
             annotations += firAnnotationList
-        } ?: FirErrorExpressionImpl(null, "Strange annotated expression: ${firExpression?.render()}")
+        } ?: FirErrorExpressionImpl(null, FirSimpleDiagnostic("Strange annotated expression: ${firExpression?.render()}"))
     }
 
     /**
@@ -355,7 +356,7 @@ class ExpressionsConverter(
      * @see org.jetbrains.kotlin.fir.builder.RawFirBuilder.Visitor.visitClassLiteralExpression
      */
     private fun convertClassLiteralExpression(classLiteralExpression: LighterASTNode): FirExpression {
-        var firReceiverExpression: FirExpression = FirErrorExpressionImpl(null, "No receiver in class literal")
+        var firReceiverExpression: FirExpression = FirErrorExpressionImpl(null, FirSimpleDiagnostic("No receiver in class literal"))
         classLiteralExpression.forEachChildren {
             if (it.isExpression()) firReceiverExpression = getAsFirExpression(it, "No receiver in class literal")
         }
@@ -399,7 +400,7 @@ class ExpressionsConverter(
     private fun convertQualifiedExpression(dotQualifiedExpression: LighterASTNode): FirExpression {
         var isSelector = false
         var isSafe = false
-        var firSelector: FirExpression = FirErrorExpressionImpl(null, "Qualified expression without selector") //after dot
+        var firSelector: FirExpression = FirErrorExpressionImpl(null, FirSimpleDiagnostic("Qualified expression without selector")) //after dot
         var firReceiver: FirExpression? = null //before dot
         dotQualifiedExpression.forEachChildren {
             when (it.tokenType) {
@@ -449,7 +450,7 @@ class ExpressionsConverter(
                     arguments += additionalArgument!!
                     FirSimpleNamedReference(null, OperatorNameConventions.INVOKE, null)
                 }
-                else -> FirErrorNamedReferenceImpl(null, "Call has no callee")
+                else -> FirErrorNamedReferenceImpl(null, FirSimpleDiagnostic("Call has no callee"))
             }
 
             context.firFunctionCalls += this
@@ -467,7 +468,7 @@ class ExpressionsConverter(
     }
 
     private fun LighterASTNode?.convertShortOrLongStringTemplate(errorReason: String): FirExpression {
-        var firExpression: FirExpression = FirErrorExpressionImpl(null, errorReason)
+        var firExpression: FirExpression = FirErrorExpressionImpl(null, FirSimpleDiagnostic(errorReason))
         this?.forEachChildren(LONG_TEMPLATE_ENTRY_START, LONG_TEMPLATE_ENTRY_END) {
             firExpression = getAsFirExpression(it, errorReason)
         }
@@ -563,7 +564,7 @@ class ExpressionsConverter(
     }
 
     private fun convertWhenConditionExpression(whenCondition: LighterASTNode): FirExpression {
-        var firExpression: FirExpression = FirErrorExpressionImpl(null, "No expression in condition with expression")
+        var firExpression: FirExpression = FirErrorExpressionImpl(null, FirSimpleDiagnostic("No expression in condition with expression"))
         whenCondition.forEachChildren {
             when (it.tokenType) {
                 else -> if (it.isExpression()) firExpression = getAsFirExpression(it, "No expression in condition with expression")
@@ -577,7 +578,7 @@ class ExpressionsConverter(
 
     private fun convertWhenConditionInRange(whenCondition: LighterASTNode): FirExpression {
         var isNegate = false
-        var firExpression: FirExpression = FirErrorExpressionImpl(null, "No range in condition with range")
+        var firExpression: FirExpression = FirErrorExpressionImpl(null, FirSimpleDiagnostic("No range in condition with range"))
         whenCondition.forEachChildren {
             when (it.tokenType) {
                 NOT_IN -> isNegate = true
@@ -611,7 +612,7 @@ class ExpressionsConverter(
      * @see org.jetbrains.kotlin.fir.builder.RawFirBuilder.Visitor.visitArrayAccessExpression
      */
     private fun convertArrayAccessExpression(arrayAccess: LighterASTNode): FirFunctionCall {
-        var firExpression: FirExpression = FirErrorExpressionImpl(null, "No array expression")
+        var firExpression: FirExpression = FirErrorExpressionImpl(null, FirSimpleDiagnostic("No array expression"))
         val indices: MutableList<FirExpression> = mutableListOf()
         arrayAccess.forEachChildren {
             when (it.tokenType) {
@@ -669,7 +670,7 @@ class ExpressionsConverter(
      */
     private fun convertDoWhile(doWhileLoop: LighterASTNode): FirElement {
         var block: LighterASTNode? = null
-        var firCondition: FirExpression = FirErrorExpressionImpl(null, "No condition in do-while loop")
+        var firCondition: FirExpression = FirErrorExpressionImpl(null, FirSimpleDiagnostic("No condition in do-while loop"))
         doWhileLoop.forEachChildren {
             when (it.tokenType) {
                 BODY -> block = it
@@ -686,7 +687,7 @@ class ExpressionsConverter(
      */
     private fun convertWhile(whileLoop: LighterASTNode): FirElement {
         var block: LighterASTNode? = null
-        var firCondition: FirExpression = FirErrorExpressionImpl(null, "No condition in while loop")
+        var firCondition: FirExpression = FirErrorExpressionImpl(null, FirSimpleDiagnostic("No condition in while loop"))
         whileLoop.forEachChildren {
             when (it.tokenType) {
                 BODY -> block = it
@@ -703,7 +704,7 @@ class ExpressionsConverter(
      */
     private fun convertFor(forLoop: LighterASTNode): FirElement {
         var parameter: ValueParameter? = null
-        var rangeExpression: FirExpression = FirErrorExpressionImpl(null, "No range in for loop")
+        var rangeExpression: FirExpression = FirErrorExpressionImpl(null, FirSimpleDiagnostic("No range in for loop"))
         var blockNode: LighterASTNode? = null
         forLoop.forEachChildren {
             when (it.tokenType) {
@@ -846,7 +847,7 @@ class ExpressionsConverter(
      * @see org.jetbrains.kotlin.fir.builder.RawFirBuilder.Visitor.visitIfExpression
      */
     private fun convertIfExpression(ifExpression: LighterASTNode): FirExpression {
-        var firCondition: FirExpression = FirErrorExpressionImpl(null, "If statement should have condition")
+        var firCondition: FirExpression = FirErrorExpressionImpl(null, FirSimpleDiagnostic("If statement should have condition"))
         var thenBlock: LighterASTNode? = null
         var elseBlock: LighterASTNode? = null
         ifExpression.forEachChildren {
@@ -908,7 +909,7 @@ class ExpressionsConverter(
      * @see org.jetbrains.kotlin.fir.builder.RawFirBuilder.Visitor.visitThrowExpression
      */
     private fun convertThrow(throwExpression: LighterASTNode): FirExpression {
-        var firExpression: FirExpression = FirErrorExpressionImpl(null, "Nothing to throw")
+        var firExpression: FirExpression = FirErrorExpressionImpl(null, FirSimpleDiagnostic("Nothing to throw"))
         throwExpression.forEachChildren {
             if (it.isExpression()) firExpression = getAsFirExpression(it, "Nothing to throw")
         }
@@ -963,7 +964,7 @@ class ExpressionsConverter(
     private fun convertValueArgument(valueArgument: LighterASTNode): FirExpression {
         var identifier: String? = null
         var isSpread = false
-        var firExpression: FirExpression = FirErrorExpressionImpl(null, "Argument is absent")
+        var firExpression: FirExpression = FirErrorExpressionImpl(null, FirSimpleDiagnostic("Argument is absent"))
         valueArgument.forEachChildren {
             when (it.tokenType) {
                 VALUE_ARGUMENT_NAME -> identifier = it.asText
diff --git a/compiler/fir/lightTree/src/org/jetbrains/kotlin/fir/lightTree/fir/ValueParameter.kt b/compiler/fir/lightTree/src/org/jetbrains/kotlin/fir/lightTree/fir/ValueParameter.kt
index 338d2bff170..c7daa222e99 100644
--- a/compiler/fir/lightTree/src/org/jetbrains/kotlin/fir/lightTree/fir/ValueParameter.kt
+++ b/compiler/fir/lightTree/src/org/jetbrains/kotlin/fir/lightTree/fir/ValueParameter.kt
@@ -12,6 +12,7 @@ import org.jetbrains.kotlin.fir.declarations.impl.FirDeclarationStatusImpl
 import org.jetbrains.kotlin.fir.declarations.impl.FirDefaultPropertyGetter
 import org.jetbrains.kotlin.fir.declarations.impl.FirDefaultPropertySetter
 import org.jetbrains.kotlin.fir.declarations.impl.FirPropertyImpl
+import org.jetbrains.kotlin.fir.diagnostics.FirSimpleDiagnostic
 import org.jetbrains.kotlin.fir.expressions.impl.FirQualifiedAccessExpressionImpl
 import org.jetbrains.kotlin.fir.lightTree.fir.modifier.Modifier
 import org.jetbrains.kotlin.fir.references.impl.FirPropertyFromParameterResolvedNamedReference
@@ -35,7 +36,7 @@ class ValueParameter(
         val name = this.firValueParameter.name
         var type = this.firValueParameter.returnTypeRef
         if (type is FirImplicitTypeRef) {
-            type = FirErrorTypeRefImpl(null, "Incomplete code")
+            type = FirErrorTypeRefImpl(null, FirSimpleDiagnostic("Incomplete code"))
         }
 
         val status = FirDeclarationStatusImpl(modifiers.getVisibility(), modifiers.getModality()).apply {
diff --git a/compiler/fir/lightTree/src/org/jetbrains/kotlin/fir/lightTree/fir/WhenEntry.kt b/compiler/fir/lightTree/src/org/jetbrains/kotlin/fir/lightTree/fir/WhenEntry.kt
index 6e083fa8d7f..06e9953e1f3 100644
--- a/compiler/fir/lightTree/src/org/jetbrains/kotlin/fir/lightTree/fir/WhenEntry.kt
+++ b/compiler/fir/lightTree/src/org/jetbrains/kotlin/fir/lightTree/fir/WhenEntry.kt
@@ -8,6 +8,7 @@ package org.jetbrains.kotlin.fir.lightTree.fir
 import org.jetbrains.kotlin.fir.FirWhenSubject
 import org.jetbrains.kotlin.fir.builder.generateContainsOperation
 import org.jetbrains.kotlin.fir.builder.generateLazyLogicalOperation
+import org.jetbrains.kotlin.fir.diagnostics.FirSimpleDiagnostic
 import org.jetbrains.kotlin.fir.expressions.*
 import org.jetbrains.kotlin.fir.expressions.impl.FirErrorExpressionImpl
 import org.jetbrains.kotlin.fir.expressions.impl.FirOperatorCallImpl
@@ -51,7 +52,7 @@ data class WhenEntry(
                 }
             }
             else -> {
-                FirErrorExpressionImpl(null, "Unsupported when condition: ${this.javaClass}")
+                FirErrorExpressionImpl(null, FirSimpleDiagnostic("Unsupported when condition: ${this.javaClass}"))
             }
         }
     }
@@ -59,7 +60,7 @@ data class WhenEntry(
     fun toFirWhenConditionWithoutSubject(): FirExpression {
         return when (val condition = conditions.first()) {
             is FirOperatorCallImpl -> condition.arguments.first()
-            else -> FirErrorExpressionImpl(null, "No expression in condition with expression")
+            else -> FirErrorExpressionImpl(null, FirSimpleDiagnostic("No expression in condition with expression"))
         }
     }
 }
\ No newline at end of file
diff --git a/compiler/fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/BaseFirBuilder.kt b/compiler/fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/BaseFirBuilder.kt
index 6c6af84a1b6..1e499e2a878 100644
--- a/compiler/fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/BaseFirBuilder.kt
+++ b/compiler/fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/BaseFirBuilder.kt
@@ -12,6 +12,7 @@ import org.jetbrains.kotlin.fir.*
 import org.jetbrains.kotlin.fir.declarations.*
 import org.jetbrains.kotlin.fir.declarations.impl.FirErrorFunctionImpl
 import org.jetbrains.kotlin.fir.declarations.impl.FirTypeParameterImpl
+import org.jetbrains.kotlin.fir.diagnostics.FirSimpleDiagnostic
 import org.jetbrains.kotlin.fir.expressions.*
 import org.jetbrains.kotlin.fir.expressions.impl.*
 import org.jetbrains.kotlin.fir.references.FirReference
@@ -110,7 +111,7 @@ abstract class BaseFirBuilder<T>(val session: FirSession, val context: Context =
                         FirErrorFunctionImpl(
                             source,
                             this@BaseFirBuilder.session,
-                            "Cannot bind unlabeled return to a function",
+                            FirSimpleDiagnostic("Cannot bind unlabeled return to a function"),
                             FirErrorFunctionSymbol()
                         )
                     )
@@ -136,7 +137,7 @@ abstract class BaseFirBuilder<T>(val session: FirSession, val context: Context =
                     FirErrorFunctionImpl(
                         source,
                         this@BaseFirBuilder.session,
-                        "Cannot bind label $labelName to a function",
+                        FirSimpleDiagnostic("Cannot bind label $labelName to a function"),
                         FirErrorFunctionSymbol()
                     )
                 )
@@ -238,7 +239,7 @@ abstract class BaseFirBuilder<T>(val session: FirSession, val context: Context =
                         expression.getSourceOrNull(), IrConstKind.Int, convertedText.toInt(), "Incorrect int: $text"
                     )
                 } else {
-                    FirErrorExpressionImpl(expression.getSourceOrNull(), reason = "Incorrect constant expression: $text")
+                    FirErrorExpressionImpl(expression.getSourceOrNull(), diagnostic = FirSimpleDiagnostic("Incorrect constant expression: $text"))
                 }
             FLOAT_CONSTANT ->
                 if (convertedText is Float) {
@@ -290,7 +291,7 @@ abstract class BaseFirBuilder<T>(val session: FirSession, val context: Context =
                 else -> {
                     hasExpressions = true
                     FirErrorExpressionImpl(
-                        entry.getSourceOrNull(), "Incorrect template entry: ${entry.asText}"
+                        entry.getSourceOrNull(), FirSimpleDiagnostic("Incorrect template entry: ${entry.asText}")
                     )
                 }
             }
@@ -343,7 +344,7 @@ abstract class BaseFirBuilder<T>(val session: FirSession, val context: Context =
         convert: T.() -> FirExpression
     ): FirExpression {
         if (argument == null) {
-            return FirErrorExpressionImpl(argument, "Inc/dec without operand")
+            return FirErrorExpressionImpl(argument, FirSimpleDiagnostic("Inc/dec without operand"))
         }
         val source = baseExpression?.toFirSourceElement()
         return FirBlockImpl(source).apply {
@@ -410,7 +411,7 @@ abstract class BaseFirBuilder<T>(val session: FirSession, val context: Context =
                         firMemberAccess.calleeReference
                     } else {
                         FirErrorNamedReferenceImpl(
-                            left.getSourceOrNull(), "Unsupported qualified LValue: ${left.asText}"
+                            left.getSourceOrNull(), FirSimpleDiagnostic("Unsupported qualified LValue: ${left.asText}")
                         )
                     }
                 }
@@ -419,7 +420,7 @@ abstract class BaseFirBuilder<T>(val session: FirSession, val context: Context =
                 }
             }
         }
-        return FirErrorNamedReferenceImpl(left.getSourceOrNull(), "Unsupported LValue: $tokenType")
+        return FirErrorNamedReferenceImpl(left.getSourceOrNull(), FirSimpleDiagnostic("Unsupported LValue: $tokenType"))
     }
 
     fun T?.generateAssignment(
@@ -468,7 +469,7 @@ abstract class BaseFirBuilder<T>(val session: FirSession, val context: Context =
                 statements += generateTemporaryVariable(
                     this@BaseFirBuilder.session, this@generateAssignment.getSourceOrNull(), name,
                     this@generateAssignment?.convert()
-                        ?: FirErrorExpressionImpl(this.getSourceOrNull(), "No LValue in assignment")
+                        ?: FirErrorExpressionImpl(this.getSourceOrNull(), FirSimpleDiagnostic("No LValue in assignment"))
                 )
                 statements += FirVariableAssignmentImpl(source, false, value, operation).apply {
                     lValue = FirSimpleNamedReference(this.getSourceOrNull(), name, null)
@@ -479,7 +480,7 @@ abstract class BaseFirBuilder<T>(val session: FirSession, val context: Context =
         if (operation in FirOperation.ASSIGNMENTS && operation != FirOperation.ASSIGN) {
             return FirOperatorCallImpl(source, operation).apply {
                 // TODO: take good psi
-                arguments += this@generateAssignment?.convert() ?: FirErrorExpressionImpl(null, "Unsupported left value of assignment: ${source?.psi?.text}")
+                arguments += this@generateAssignment?.convert() ?: FirErrorExpressionImpl(null, FirSimpleDiagnostic("Unsupported left value of assignment: ${source?.psi?.text}"))
                 arguments += value
             }
         }
diff --git a/compiler/fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/ConversionUtils.kt b/compiler/fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/ConversionUtils.kt
index 639efb8af7c..1faed478de9 100644
--- a/compiler/fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/ConversionUtils.kt
+++ b/compiler/fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/ConversionUtils.kt
@@ -11,6 +11,7 @@ import org.jetbrains.kotlin.descriptors.Visibilities
 import org.jetbrains.kotlin.fir.*
 import org.jetbrains.kotlin.fir.declarations.FirVariable
 import org.jetbrains.kotlin.fir.declarations.impl.*
+import org.jetbrains.kotlin.fir.diagnostics.FirSimpleDiagnostic
 import org.jetbrains.kotlin.fir.expressions.*
 import org.jetbrains.kotlin.fir.expressions.impl.*
 import org.jetbrains.kotlin.fir.impl.FirAbstractAnnotatedElement
@@ -200,7 +201,7 @@ internal fun KtWhenCondition.toFirWhenCondition(
             }
         }
         else -> {
-            FirErrorExpressionImpl(this.toFirSourceElement(), "Unsupported when condition: ${this.javaClass}")
+            FirErrorExpressionImpl(this.toFirSourceElement(), FirSimpleDiagnostic("Unsupported when condition: ${this.javaClass}"))
         }
     }
 }
diff --git a/compiler/fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/RawFirBuilder.kt b/compiler/fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/RawFirBuilder.kt
index b69fe4c17a2..e5f9ecb7c58 100644
--- a/compiler/fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/RawFirBuilder.kt
+++ b/compiler/fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/RawFirBuilder.kt
@@ -14,6 +14,7 @@ import org.jetbrains.kotlin.descriptors.Visibility
 import org.jetbrains.kotlin.fir.*
 import org.jetbrains.kotlin.fir.declarations.*
 import org.jetbrains.kotlin.fir.declarations.impl.*
+import org.jetbrains.kotlin.fir.diagnostics.FirSimpleDiagnostic
 import org.jetbrains.kotlin.fir.expressions.*
 import org.jetbrains.kotlin.fir.expressions.impl.*
 import org.jetbrains.kotlin.fir.impl.FirAbstractAnnotatedElement
@@ -105,21 +106,21 @@ class RawFirBuilder(session: FirSession, val stubMode: Boolean) : BaseFirBuilder
             convertSafe() ?: implicitUnitType
 
         private fun KtTypeReference?.toFirOrErrorType(): FirTypeRef =
-            convertSafe() ?: FirErrorTypeRefImpl(this?.toFirSourceElement(), if (this == null) "Incomplete code" else "Conversion failed")
+            convertSafe() ?: FirErrorTypeRefImpl(this?.toFirSourceElement(), FirSimpleDiagnostic(if (this == null) "Incomplete code" else "Conversion failed"))
 
         // Here we accept lambda as receiver to prevent expression calculation in stub mode
         private fun (() -> KtExpression?).toFirExpression(errorReason: String): FirExpression =
             if (stubMode) FirExpressionStub(null)
             else with(this()) {
-                convertSafe<FirExpression>() ?: FirErrorExpressionImpl(this?.toFirSourceElement(), errorReason)
+                convertSafe<FirExpression>() ?: FirErrorExpressionImpl(this?.toFirSourceElement(), FirSimpleDiagnostic(errorReason))
             }
 
         private fun KtExpression?.toFirExpression(errorReason: String): FirExpression =
             if (stubMode) FirExpressionStub(null)
-            else convertSafe<FirExpression>() ?: FirErrorExpressionImpl(this?.toFirSourceElement(), errorReason)
+            else convertSafe<FirExpression>() ?: FirErrorExpressionImpl(this?.toFirSourceElement(), FirSimpleDiagnostic(errorReason))
 
         private fun KtExpression.toFirStatement(errorReason: String): FirStatement =
-            convertSafe() ?: FirErrorExpressionImpl(this.toFirSourceElement(), errorReason)
+            convertSafe() ?: FirErrorExpressionImpl(this.toFirSourceElement(), FirSimpleDiagnostic(errorReason))
 
         private fun KtExpression.toFirStatement(): FirStatement =
             convert()
@@ -130,7 +131,7 @@ class RawFirBuilder(session: FirSession, val stubMode: Boolean) : BaseFirBuilder
             return when (this) {
                 is KtSecondaryConstructor -> toFirConstructor(
                     delegatedSuperType,
-                    delegatedSelfType ?: FirErrorTypeRefImpl(this.toFirSourceElement(), "Constructor in object"),
+                    delegatedSelfType ?: FirErrorTypeRefImpl(this.toFirSourceElement(), FirSimpleDiagnostic("Constructor in object")),
                     owner,
                     hasPrimaryConstructor
                 )
@@ -165,7 +166,7 @@ class RawFirBuilder(session: FirSession, val stubMode: Boolean) : BaseFirBuilder
             }
 
         private fun ValueArgument?.toFirExpression(): FirExpression {
-            this ?: return FirErrorExpressionImpl((this as? KtElement)?.toFirSourceElement(), "No argument given")
+            this ?: return FirErrorExpressionImpl((this as? KtElement)?.toFirSourceElement(), FirSimpleDiagnostic("No argument given"))
             val name = this.getArgumentName()?.asName
             val expression = this.getArgumentExpression()
             val firExpression = when (expression) {
@@ -718,7 +719,7 @@ class RawFirBuilder(session: FirSession, val stubMode: Boolean) : BaseFirBuilder
             val source = this.toFirSourceElement()
             val delegatedType = when {
                 isThis -> delegatedSelfTypeRef
-                else -> delegatedSuperTypeRef ?: FirErrorTypeRefImpl(source, "No super type")
+                else -> delegatedSuperTypeRef ?: FirErrorTypeRefImpl(source, FirSimpleDiagnostic("No super type"))
             }
             return FirDelegatedConstructorCallImpl(
                 source,
@@ -837,7 +838,7 @@ class RawFirBuilder(session: FirSession, val stubMode: Boolean) : BaseFirBuilder
 
                         userType
                     } else {
-                        FirErrorTypeRefImpl(source, "Incomplete user type")
+                        FirErrorTypeRefImpl(source, FirSimpleDiagnostic("Incomplete user type"))
                     }
                 }
                 is KtFunctionType -> {
@@ -856,7 +857,7 @@ class RawFirBuilder(session: FirSession, val stubMode: Boolean) : BaseFirBuilder
                     }
                     functionType
                 }
-                null -> FirErrorTypeRefImpl(source, "Unwrapped type is null")
+                null -> FirErrorTypeRefImpl(source, FirSimpleDiagnostic("Unwrapped type is null"))
                 else -> throw AssertionError("Unexpected type element: ${unwrappedElement.text}")
             }
 
@@ -1230,7 +1231,7 @@ class RawFirBuilder(session: FirSession, val stubMode: Boolean) : BaseFirBuilder
                         calleeExpression.toFirSourceElement(), calleeExpression.getReferencedNameAsName(), null
                     )
                     null -> FirErrorNamedReferenceImpl(
-                        null, "Call has no callee"
+                        null, FirSimpleDiagnostic("Call has no callee")
                     )
                     else -> {
                         arguments += calleeExpression.toFirExpression("Incorrect invoke receiver")
@@ -1263,7 +1264,7 @@ class RawFirBuilder(session: FirSession, val stubMode: Boolean) : BaseFirBuilder
 
         override fun visitQualifiedExpression(expression: KtQualifiedExpression, data: Unit): FirElement {
             val selector = expression.selectorExpression
-                ?: return FirErrorExpressionImpl(expression.toFirSourceElement(), "Qualified expression without selector")
+                ?: return FirErrorExpressionImpl(expression.toFirSourceElement(), FirSimpleDiagnostic("Qualified expression without selector"))
             val firSelector = selector.toFirExpression("Incorrect selector expression")
             if (firSelector is FirModifiableQualifiedAccess) {
                 firSelector.safe = expression is KtSafeQualifiedExpression
@@ -1287,7 +1288,7 @@ class RawFirBuilder(session: FirSession, val stubMode: Boolean) : BaseFirBuilder
         }
 
         override fun visitParenthesizedExpression(expression: KtParenthesizedExpression, data: Unit): FirElement {
-            return expression.expression?.accept(this, data) ?: FirErrorExpressionImpl(expression.toFirSourceElement(), "Empty parentheses")
+            return expression.expression?.accept(this, data) ?: FirErrorExpressionImpl(expression.toFirSourceElement(), FirSimpleDiagnostic("Empty parentheses"))
         }
 
         override fun visitLabeledExpression(expression: KtLabeledExpression, data: Unit): FirElement {
@@ -1297,7 +1298,7 @@ class RawFirBuilder(session: FirSession, val stubMode: Boolean) : BaseFirBuilder
             if (labelName != null) {
                 context.firLabels += FirLabelImpl(source, labelName)
             }
-            val result = expression.baseExpression?.accept(this, data) ?: FirErrorExpressionImpl(source, "Empty label")
+            val result = expression.baseExpression?.accept(this, data) ?: FirErrorExpressionImpl(source, FirSimpleDiagnostic("Empty label"))
             if (size != context.firLabels.size) {
                 context.firLabels.removeLast()
                 println("Unused label: ${expression.text}")
@@ -1308,7 +1309,7 @@ class RawFirBuilder(session: FirSession, val stubMode: Boolean) : BaseFirBuilder
         override fun visitAnnotatedExpression(expression: KtAnnotatedExpression, data: Unit): FirElement {
             val rawResult = expression.baseExpression?.accept(this, data)
             val result = rawResult as? FirAbstractAnnotatedElement
-                ?: FirErrorExpressionImpl(expression.toFirSourceElement(), "Strange annotated expression: ${rawResult?.render()}")
+                ?: FirErrorExpressionImpl(expression.toFirSourceElement(), FirSimpleDiagnostic("Strange annotated expression: ${rawResult?.render()}"))
             expression.extractAnnotationsTo(result)
             return result
         }
diff --git a/compiler/fir/psi2fir/tests/org/jetbrains/kotlin/fir/builder/RawFirBuilderTotalKotlinTestCase.kt b/compiler/fir/psi2fir/tests/org/jetbrains/kotlin/fir/builder/RawFirBuilderTotalKotlinTestCase.kt
index 67c51c77dc1..e3c201e3d90 100644
--- a/compiler/fir/psi2fir/tests/org/jetbrains/kotlin/fir/builder/RawFirBuilderTotalKotlinTestCase.kt
+++ b/compiler/fir/psi2fir/tests/org/jetbrains/kotlin/fir/builder/RawFirBuilderTotalKotlinTestCase.kt
@@ -76,7 +76,7 @@ class RawFirBuilderTotalKotlinTestCase : AbstractRawFirBuilderTestCase() {
                         val calleeReference = qualifiedAccess.calleeReference
                         if (calleeReference is FirErrorNamedReference) {
                             errorReferences++
-                            println(calleeReference.errorReason)
+                            println(calleeReference.diagnostic.reason)
                         } else {
                             normalReferences++
                         }
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/FirCallResolver.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/FirCallResolver.kt
index 988e07c6cc3..15952568fb0 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/FirCallResolver.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/FirCallResolver.kt
@@ -7,6 +7,7 @@ package org.jetbrains.kotlin.fir
 
 import org.jetbrains.kotlin.fir.declarations.FirFile
 import org.jetbrains.kotlin.fir.declarations.FirMemberDeclaration
+import org.jetbrains.kotlin.fir.diagnostics.FirSimpleDiagnostic
 import org.jetbrains.kotlin.fir.expressions.*
 import org.jetbrains.kotlin.fir.expressions.impl.FirExpressionStub
 import org.jetbrains.kotlin.fir.expressions.impl.FirResolvedQualifierImpl
@@ -365,12 +366,12 @@ class FirCallResolver(
         val source = namedReference.source
         return when {
             candidates.isEmpty() -> FirErrorNamedReferenceImpl(
-                source, "Unresolved name: $name"
+                source, FirSimpleDiagnostic("Unresolved name: $name")
             )
             applicability < CandidateApplicability.SYNTHETIC_RESOLVED -> {
                 FirErrorNamedReferenceImpl(
                     source,
-                    "Inapplicable($applicability): ${candidates.map { describeSymbol(it.symbol) }}"
+                    FirSimpleDiagnostic("Inapplicable($applicability): ${candidates.map { describeSymbol(it.symbol) }}")
                 )
             }
             candidates.size == 1 -> {
@@ -387,7 +388,7 @@ class FirCallResolver(
                 }
             }
             else -> FirErrorNamedReferenceImpl(
-                source, "Ambiguity: $name, ${candidates.map { describeSymbol(it.symbol) }}"
+                source, FirSimpleDiagnostic("Ambiguity: $name, ${candidates.map { describeSymbol(it.symbol) }}")
             )
         }
     }
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/deserialization/AbstractAnnotationDeserializer.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/deserialization/AbstractAnnotationDeserializer.kt
index 54041ee0998..5ace4df74b9 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/deserialization/AbstractAnnotationDeserializer.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/deserialization/AbstractAnnotationDeserializer.kt
@@ -8,6 +8,7 @@ package org.jetbrains.kotlin.fir.deserialization
 import org.jetbrains.kotlin.descriptors.ClassKind
 import org.jetbrains.kotlin.fir.FirSession
 import org.jetbrains.kotlin.fir.declarations.FirConstructor
+import org.jetbrains.kotlin.fir.diagnostics.FirSimpleDiagnostic
 import org.jetbrains.kotlin.fir.expressions.FirAnnotationCall
 import org.jetbrains.kotlin.fir.expressions.FirExpression
 import org.jetbrains.kotlin.fir.expressions.impl.*
@@ -96,7 +97,7 @@ abstract class AbstractAnnotationDeserializer(
                 FirResolvedTypeRefImpl(
                     null, it.constructType(emptyList(), isNullable = false)
                 )
-            } ?: FirErrorTypeRefImpl(null, "Symbol not found for $classId")
+            } ?: FirErrorTypeRefImpl(null, FirSimpleDiagnostic("Symbol not found for $classId"))
         ).apply {
             this.arguments += arguments
         }
@@ -137,7 +138,7 @@ abstract class AbstractAnnotationDeserializer(
                     FirResolvedNamedReferenceImpl(null, entryName, it)
                 } ?: FirErrorNamedReferenceImpl(
                     null,
-                    errorReason = "Strange deserialized enum value: $classId.$entryName"
+                    FirSimpleDiagnostic("Strange deserialized enum value: $classId.$entryName")
                 )
             }
 //            ARRAY -> {
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/ResolveUtils.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/ResolveUtils.kt
index 62ed7a461c6..c9379214f75 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/ResolveUtils.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/ResolveUtils.kt
@@ -9,6 +9,7 @@ import org.jetbrains.kotlin.descriptors.ClassKind
 import org.jetbrains.kotlin.fir.FirSession
 import org.jetbrains.kotlin.fir.componentArrayAccessor
 import org.jetbrains.kotlin.fir.declarations.*
+import org.jetbrains.kotlin.fir.diagnostics.FirSimpleDiagnostic
 import org.jetbrains.kotlin.fir.expressions.FirQualifiedAccess
 import org.jetbrains.kotlin.fir.expressions.FirResolvable
 import org.jetbrains.kotlin.fir.expressions.FirResolvedQualifier
@@ -295,7 +296,7 @@ fun <T : FirResolvable> BodyResolveComponents.typeFromCallee(access: T): FirReso
 
     return when (val newCallee = access.calleeReference) {
         is FirErrorNamedReference ->
-            FirErrorTypeRefImpl(access.source, newCallee.errorReason)
+            FirErrorTypeRefImpl(access.source, newCallee.diagnostic)
         is FirNamedReferenceWithCandidate -> {
             typeFromSymbol(newCallee.candidateSymbol, makeNullable)
         }
@@ -329,7 +330,7 @@ private fun BodyResolveComponents.typeFromSymbol(symbol: AbstractFirBasedSymbol<
             if (fir is FirEnumEntry) {
                 (fir.superTypeRefs.firstOrNull() as? FirResolvedTypeRef) ?: FirErrorTypeRefImpl(
                     null,
-                    "no enum item supertype"
+                    FirSimpleDiagnostic("no enum item supertype")
                 )
             } else
                 FirResolvedTypeRefImpl(
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/PostponedArgumentsAnalyzer.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/PostponedArgumentsAnalyzer.kt
index 1e3c731c1c3..e242d528692 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/PostponedArgumentsAnalyzer.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/PostponedArgumentsAnalyzer.kt
@@ -7,6 +7,7 @@ package org.jetbrains.kotlin.fir.resolve.calls
 
 import org.jetbrains.kotlin.fir.FirCallResolver
 import org.jetbrains.kotlin.fir.declarations.FirAnonymousFunction
+import org.jetbrains.kotlin.fir.diagnostics.FirSimpleDiagnostic
 import org.jetbrains.kotlin.fir.expressions.FirExpression
 import org.jetbrains.kotlin.fir.references.impl.FirErrorNamedReferenceImpl
 import org.jetbrains.kotlin.fir.resolve.constructType
@@ -84,7 +85,7 @@ class PostponedArgumentsAnalyzer(
             candidate == null || applicability < CandidateApplicability.SYNTHETIC_RESOLVED ->
                 FirErrorNamedReferenceImpl(
                     callableReferenceAccess.source,
-                    "Unresolved reference: ${callableReferenceAccess.calleeReference.name}"
+                    FirSimpleDiagnostic("Unresolved reference: ${callableReferenceAccess.calleeReference.name}")
                 )
             else -> FirNamedReferenceWithCandidate(callableReferenceAccess.source, callableReferenceAccess.calleeReference.name, candidate)
         }
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/impl/FirTypeResolverImpl.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/impl/FirTypeResolverImpl.kt
index 4b682c2e8e2..bafeb6f70da 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/impl/FirTypeResolverImpl.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/impl/FirTypeResolverImpl.kt
@@ -104,9 +104,7 @@ class FirTypeResolverImpl(private val session: FirSession) : FirTypeResolver {
             is FirUserTypeRef -> {
                 resolveUserType(typeRef, resolveToSymbol(typeRef, scope), scope)
             }
-            is FirErrorTypeRef -> {
-                ConeKotlinErrorType(typeRef.reason)
-            }
+            is FirErrorTypeRef -> typeRef.type
             is FirFunctionTypeRef -> {
                 createFunctionalType(typeRef)
             }
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/FirSupertypeResolverTransformer.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/FirSupertypeResolverTransformer.kt
index 8bfe410deeb..ec38a866ef3 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/FirSupertypeResolverTransformer.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/FirSupertypeResolverTransformer.kt
@@ -7,6 +7,7 @@ package org.jetbrains.kotlin.fir.resolve.transformers
 
 import org.jetbrains.kotlin.fir.FirSession
 import org.jetbrains.kotlin.fir.declarations.*
+import org.jetbrains.kotlin.fir.diagnostics.FirSimpleDiagnostic
 import org.jetbrains.kotlin.fir.expressions.FirStatement
 import org.jetbrains.kotlin.fir.render
 import org.jetbrains.kotlin.fir.resolve.ScopeSession
@@ -164,7 +165,7 @@ class FirSupertypeResolverTransformer : FirAbstractTreeTransformer(phase = FirRe
                 val coneType = (superTypeRef as FirResolvedTypeRef).type
                 if (coneType is ConeTypeParameterType) {
                     resultingTypeRefs.add(
-                        FirErrorTypeRefImpl(superTypeRef.source, "Type parameter cannot be a super-type: ${coneType.render()}")
+                        FirErrorTypeRefImpl(superTypeRef.source, FirSimpleDiagnostic("Type parameter cannot be a super-type: ${coneType.render()}"))
                     )
                     continue
                 }
@@ -173,7 +174,7 @@ class FirSupertypeResolverTransformer : FirAbstractTreeTransformer(phase = FirRe
 
                 if (superTypeClassId.outerClasses().any { it.areSupertypesComputing() }) {
                     resultingTypeRefs.add(
-                        FirErrorTypeRefImpl(superTypeRef.source, "Recursion detected: ${superTypeRef.render()}")
+                        FirErrorTypeRefImpl(superTypeRef.source, FirSimpleDiagnostic("Recursion detected: ${superTypeRef.render()}"))
                     )
 
                     continue
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/ReturnTypeCalculator.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/ReturnTypeCalculator.kt
index a4e4936406a..c793224a4d0 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/ReturnTypeCalculator.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/ReturnTypeCalculator.kt
@@ -10,6 +10,7 @@ import org.jetbrains.kotlin.fir.FirSession
 import org.jetbrains.kotlin.fir.declarations.FirCallableMemberDeclaration
 import org.jetbrains.kotlin.fir.declarations.FirTypedDeclaration
 import org.jetbrains.kotlin.fir.declarations.FirValueParameter
+import org.jetbrains.kotlin.fir.diagnostics.FirSimpleDiagnostic
 import org.jetbrains.kotlin.fir.render
 import org.jetbrains.kotlin.fir.resolve.ScopeSession
 import org.jetbrains.kotlin.fir.resolve.firProvider
@@ -31,7 +32,7 @@ class ReturnTypeCalculatorWithJump(val session: FirSession, val scopeSession: Sc
         if (declaration.returnTypeRef is FirComputingImplicitTypeRef) {
             declaration.transformReturnTypeRef(
                 TransformImplicitType,
-                FirErrorTypeRefImpl(null, "cycle")
+                FirErrorTypeRefImpl(null, FirSimpleDiagnostic("cycle"))
             )
             return declaration.returnTypeRef as FirResolvedTypeRef
         }
@@ -46,7 +47,7 @@ class ReturnTypeCalculatorWithJump(val session: FirSession, val scopeSession: Sc
                 TransformImplicitType,
                 FirErrorTypeRefImpl(
                     null,
-                    "Unsupported: implicit VP type"
+                    FirSimpleDiagnostic("Unsupported: implicit VP type")
                 )
             )
         }
@@ -68,7 +69,7 @@ class ReturnTypeCalculatorWithJump(val session: FirSession, val scopeSession: Sc
         }.mapTo(mutableListOf()) { provider.getFirClassifierByFqName(it) }
 
         if (file == null || outerClasses.any { it == null }) {
-            return FirErrorTypeRefImpl(null, "Cannot calculate return type (local class/object?)")
+            return FirErrorTypeRefImpl(null, FirSimpleDiagnostic("Cannot calculate return type (local class/object?)"))
         }
 
         declaration.transformReturnTypeRef(
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/body/resolve/FirControlFlowStatementsResolveTransformer.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/body/resolve/FirControlFlowStatementsResolveTransformer.kt
index c1b7e1edda1..4f985dc4252 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/body/resolve/FirControlFlowStatementsResolveTransformer.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/body/resolve/FirControlFlowStatementsResolveTransformer.kt
@@ -6,6 +6,7 @@
 package org.jetbrains.kotlin.fir.resolve.transformers.body.resolve
 
 import org.jetbrains.kotlin.fir.FirTargetElement
+import org.jetbrains.kotlin.fir.diagnostics.FirSimpleDiagnostic
 import org.jetbrains.kotlin.fir.expressions.*
 import org.jetbrains.kotlin.fir.expressions.impl.FirElseIfTrueCondition
 import org.jetbrains.kotlin.fir.expressions.impl.FirEmptyExpressionBlock
@@ -71,7 +72,7 @@ class FirControlFlowStatementsResolveTransformer(transformer: FirBodyResolveTran
                     whenExpression = syntheticCallGenerator.generateCalleeForWhenExpression(whenExpression) ?: run {
                         whenExpression = whenExpression.transformSingle(whenExhaustivenessTransformer, null)
                         dataFlowAnalyzer.exitWhenExpression(whenExpression)
-                        whenExpression.resultType = FirErrorTypeRefImpl(null, "")
+                        whenExpression.resultType = FirErrorTypeRefImpl(null, FirSimpleDiagnostic(""))
                         return@with whenExpression.compose()
                     }
 
@@ -136,7 +137,7 @@ class FirControlFlowStatementsResolveTransformer(transformer: FirBodyResolveTran
             val expectedTypeRef = data as FirTypeRef?
             callCompleter.completeCall(it, expectedTypeRef)
         } ?: run {
-            tryExpression.resultType = FirErrorTypeRefImpl(null, "")
+            tryExpression.resultType = FirErrorTypeRefImpl(null, FirSimpleDiagnostic(""))
             tryExpression
         }
 
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/body/resolve/FirDeclarationsResolveTransformer.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/body/resolve/FirDeclarationsResolveTransformer.kt
index 4d9c09627d2..7f550d0e926 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/body/resolve/FirDeclarationsResolveTransformer.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/body/resolve/FirDeclarationsResolveTransformer.kt
@@ -10,6 +10,7 @@ import org.jetbrains.kotlin.fir.copy
 import org.jetbrains.kotlin.fir.declarations.*
 import org.jetbrains.kotlin.fir.declarations.impl.FirDefaultPropertyAccessor
 import org.jetbrains.kotlin.fir.declarations.impl.FirValueParameterImpl
+import org.jetbrains.kotlin.fir.diagnostics.FirSimpleDiagnostic
 import org.jetbrains.kotlin.fir.expressions.FirStatement
 import org.jetbrains.kotlin.fir.resolve.calls.ImplicitDispatchReceiverValue
 import org.jetbrains.kotlin.fir.resolve.calls.ImplicitExtensionReceiverValue
@@ -336,7 +337,7 @@ class FirDeclarationsResolveTransformer(transformer: FirBodyResolveTransformer)
                 val bodyExpectedType = returnTypeRefFromResolvedAtom ?: data
                 af = transformFunction(af, bodyExpectedType).single as FirAnonymousFunction
                 af = af.copy(
-                    returnTypeRef = af.body?.resultType ?: FirErrorTypeRefImpl(af.source, "No result type for lambda")
+                    returnTypeRef = af.body?.resultType ?: FirErrorTypeRefImpl(af.source, FirSimpleDiagnostic("No result type for lambda"))
                 )
                 af.replaceTypeRef(af.constructFunctionalTypeRef(session))
                 af.compose()
@@ -383,7 +384,7 @@ class FirDeclarationsResolveTransformer(transformer: FirBodyResolveTransformer)
                         when (val resultType = initializer.resultType) {
                             is FirImplicitTypeRef -> FirErrorTypeRefImpl(
                                 null,
-                                "No result type for initializer"
+                                FirSimpleDiagnostic("No result type for initializer")
                             )
                             else -> resultType
                         }
@@ -395,7 +396,7 @@ class FirDeclarationsResolveTransformer(transformer: FirBodyResolveTransformer)
                         when (val resultType = variable.getter?.returnTypeRef) {
                             is FirImplicitTypeRef -> FirErrorTypeRefImpl(
                                 null,
-                                "No result type for getter"
+                                FirSimpleDiagnostic("No result type for getter")
                             )
                             else -> resultType
                         }
@@ -403,7 +404,7 @@ class FirDeclarationsResolveTransformer(transformer: FirBodyResolveTransformer)
                 }
                 else -> {
                     variable.transformReturnTypeRef(
-                        transformer, FirErrorTypeRefImpl(null, "Cannot infer variable type without initializer / getter / delegate")
+                        transformer, FirErrorTypeRefImpl(null, FirSimpleDiagnostic("Cannot infer variable type without initializer / getter / delegate"))
                     )
                 }
             }
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/body/resolve/FirExpressionsResolveTransformer.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/body/resolve/FirExpressionsResolveTransformer.kt
index 55f3f01fd06..a49b62e5e2b 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/body/resolve/FirExpressionsResolveTransformer.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/body/resolve/FirExpressionsResolveTransformer.kt
@@ -8,6 +8,7 @@ package org.jetbrains.kotlin.fir.resolve.transformers.body.resolve
 import org.jetbrains.kotlin.fir.BuiltinTypes
 import org.jetbrains.kotlin.fir.FirCallResolver
 import org.jetbrains.kotlin.fir.declarations.FirTypeParametersOwner
+import org.jetbrains.kotlin.fir.diagnostics.FirSimpleDiagnostic
 import org.jetbrains.kotlin.fir.expressions.*
 import org.jetbrains.kotlin.fir.expressions.impl.FirErrorExpressionImpl
 import org.jetbrains.kotlin.fir.expressions.impl.FirExpressionWithSmartcastImpl
@@ -55,7 +56,7 @@ class FirExpressionsResolveTransformer(transformer: FirBodyResolveTransformer) :
 
     override fun transformExpression(expression: FirExpression, data: Any?): CompositeTransformResult<FirStatement> {
         if (expression.resultType is FirImplicitTypeRef && expression !is FirWrappedExpression) {
-            val type = FirErrorTypeRefImpl(expression.source, "Type calculating for ${expression::class} is not supported")
+            val type = FirErrorTypeRefImpl(expression.source, FirSimpleDiagnostic("Type calculating for ${expression::class} is not supported"))
             expression.resultType = type
         }
         return (expression.transformChildren(transformer, data) as FirStatement).compose()
@@ -83,7 +84,7 @@ class FirExpressionsResolveTransformer(transformer: FirBodyResolveTransformer) :
                 } else {
                     val superTypeRef = implicitReceiverStack.lastDispatchReceiver()
                         ?.boundSymbol?.phasedFir?.superTypeRefs?.firstOrNull()
-                        ?: FirErrorTypeRefImpl(qualifiedAccessExpression.source, "No super type")
+                        ?: FirErrorTypeRefImpl(qualifiedAccessExpression.source, FirSimpleDiagnostic("No super type"))
                     qualifiedAccessExpression.resultType = superTypeRef
                     callee.replaceSuperTypeRef(superTypeRef)
                 }
@@ -171,7 +172,7 @@ class FirExpressionsResolveTransformer(transformer: FirBodyResolveTransformer) :
         block.resultType = if (resultExpression == null) {
             FirImplicitUnitTypeRef(block.source)
         } else {
-            (resultExpression.resultType as? FirResolvedTypeRef) ?: FirErrorTypeRefImpl(null, "No type for block")
+            (resultExpression.resultType as? FirResolvedTypeRef) ?: FirErrorTypeRefImpl(null, FirSimpleDiagnostic("No type for block"))
         }
         dataFlowAnalyzer.exitBlock(block)
         return block.compose()
@@ -228,13 +229,13 @@ class FirExpressionsResolveTransformer(transformer: FirBodyResolveTransformer) :
                     val assignment =
                         FirVariableAssignmentImpl(operatorCall.source, false, resolvedOperatorCall, FirOperation.ASSIGN).apply {
                             lValue = (leftArgument as? FirQualifiedAccess)?.calleeReference
-                                ?: FirErrorNamedReferenceImpl(null, "Unresolved reference")
+                                ?: FirErrorNamedReferenceImpl(null, FirSimpleDiagnostic("Unresolved reference"))
                         }
                     assignment.transform(transformer, noExpectedType)
                 }
                 else -> FirErrorExpressionImpl(
                     operatorCall.source,
-                    "Operator overload ambiguity. $assignmentOperatorName and $simpleOperatorName are compatible"
+                    FirSimpleDiagnostic("Operator overload ambiguity. $assignmentOperatorName and $simpleOperatorName are compatible")
                 ).compose()
             }
         }
diff --git a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/FirRenderer.kt b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/FirRenderer.kt
index f47c1943da4..ba135606da5 100644
--- a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/FirRenderer.kt
+++ b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/FirRenderer.kt
@@ -703,7 +703,7 @@ class FirRenderer(builder: StringBuilder) : FirVisitorVoid() {
 
     override fun visitErrorTypeRef(errorTypeRef: FirErrorTypeRef) {
         visitTypeRef(errorTypeRef)
-        print("<ERROR TYPE REF: ${errorTypeRef.reason}>")
+        print("<ERROR TYPE REF: ${errorTypeRef.diagnostic.reason}>")
     }
 
     override fun visitImplicitTypeRef(implicitTypeRef: FirImplicitTypeRef) {
@@ -790,7 +790,7 @@ class FirRenderer(builder: StringBuilder) : FirVisitorVoid() {
             symbol != null -> {
                 print("R?C|${symbol.render()}|")
             }
-            namedReference is FirErrorNamedReference -> print("<${namedReference.errorReason}>#")
+            namedReference is FirErrorNamedReference -> print("<${namedReference.diagnostic.reason}>#")
             else -> print("${namedReference.name}#")
         }
     }
@@ -1000,7 +1000,7 @@ class FirRenderer(builder: StringBuilder) : FirVisitorVoid() {
     }
 
     override fun visitErrorExpression(errorExpression: FirErrorExpression) {
-        print("ERROR_EXPR(${errorExpression.reason})")
+        print("ERROR_EXPR(${errorExpression.diagnostic.reason})")
     }
 
     override fun visitResolvedQualifier(resolvedQualifier: FirResolvedQualifier) {
diff --git a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/FirErrorFunction.kt b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/FirErrorFunction.kt
index dbc7b778f82..0971e1ec15f 100644
--- a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/FirErrorFunction.kt
+++ b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/FirErrorFunction.kt
@@ -8,6 +8,8 @@ package org.jetbrains.kotlin.fir.declarations
 import org.jetbrains.kotlin.fir.FirPureAbstractElement
 import org.jetbrains.kotlin.fir.FirSession
 import org.jetbrains.kotlin.fir.FirSourceElement
+import org.jetbrains.kotlin.fir.diagnostics.FirDiagnostic
+import org.jetbrains.kotlin.fir.diagnostics.FirDiagnosticHolder
 import org.jetbrains.kotlin.fir.expressions.FirAnnotationCall
 import org.jetbrains.kotlin.fir.expressions.FirBlock
 import org.jetbrains.kotlin.fir.references.FirControlFlowGraphReference
@@ -20,7 +22,7 @@ import org.jetbrains.kotlin.fir.visitors.*
  * DO NOT MODIFY IT MANUALLY
  */
 
-abstract class FirErrorFunction : FirPureAbstractElement(), FirFunction<FirErrorFunction> {
+abstract class FirErrorFunction : FirPureAbstractElement(), FirFunction<FirErrorFunction>, FirDiagnosticHolder {
     abstract override val source: FirSourceElement?
     abstract override val session: FirSession
     abstract override val resolvePhase: FirResolvePhase
@@ -31,7 +33,7 @@ abstract class FirErrorFunction : FirPureAbstractElement(), FirFunction<FirError
     abstract override val typeParameters: List<FirTypeParameter>
     abstract override val valueParameters: List<FirValueParameter>
     abstract override val body: FirBlock?
-    abstract val reason: String
+    abstract override val diagnostic: FirDiagnostic
     abstract override val symbol: FirErrorFunctionSymbol
 
     override fun <R, D> accept(visitor: FirVisitor<R, D>, data: D): R = visitor.visitErrorFunction(this, data)
diff --git a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/impl/FirErrorFunctionImpl.kt b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/impl/FirErrorFunctionImpl.kt
index e6a826fb7b6..08dc295003a 100644
--- a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/impl/FirErrorFunctionImpl.kt
+++ b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/impl/FirErrorFunctionImpl.kt
@@ -11,6 +11,7 @@ import org.jetbrains.kotlin.fir.declarations.FirErrorFunction
 import org.jetbrains.kotlin.fir.declarations.FirResolvePhase
 import org.jetbrains.kotlin.fir.declarations.FirTypeParameter
 import org.jetbrains.kotlin.fir.declarations.FirValueParameter
+import org.jetbrains.kotlin.fir.diagnostics.FirDiagnostic
 import org.jetbrains.kotlin.fir.expressions.FirAnnotationCall
 import org.jetbrains.kotlin.fir.expressions.FirBlock
 import org.jetbrains.kotlin.fir.impl.FirAbstractAnnotatedElement
@@ -29,12 +30,12 @@ import org.jetbrains.kotlin.fir.visitors.*
 class FirErrorFunctionImpl(
     override val source: FirSourceElement?,
     override val session: FirSession,
-    override val reason: String,
+    override val diagnostic: FirDiagnostic,
     override val symbol: FirErrorFunctionSymbol
 ) : FirErrorFunction(), FirAbstractAnnotatedElement {
     override var resolvePhase: FirResolvePhase = FirResolvePhase.RAW_FIR
     override val annotations: MutableList<FirAnnotationCall> = mutableListOf()
-    override var returnTypeRef: FirTypeRef = FirErrorTypeRefImpl(null, reason)
+    override var returnTypeRef: FirTypeRef = FirErrorTypeRefImpl(null, diagnostic)
     override val receiverTypeRef: FirTypeRef? get() = null
     override var controlFlowGraphReference: FirControlFlowGraphReference = FirEmptyControlFlowGraphReference()
     override val typeParameters: MutableList<FirTypeParameter> = mutableListOf()
diff --git a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/diagnostics/FirDiagnostic.kt b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/diagnostics/FirDiagnostic.kt
new file mode 100644
index 00000000000..674c7153e24
--- /dev/null
+++ b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/diagnostics/FirDiagnostic.kt
@@ -0,0 +1,10 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.fir.diagnostics
+
+abstract class FirDiagnostic {
+    abstract val reason: String
+}
\ No newline at end of file
diff --git a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/diagnostics/FirDiagnosticHolder.kt b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/diagnostics/FirDiagnosticHolder.kt
new file mode 100644
index 00000000000..4c7aa5577d0
--- /dev/null
+++ b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/diagnostics/FirDiagnosticHolder.kt
@@ -0,0 +1,22 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.fir.diagnostics
+
+import org.jetbrains.kotlin.fir.FirElement
+import org.jetbrains.kotlin.fir.FirSourceElement
+import org.jetbrains.kotlin.fir.visitors.*
+
+/*
+ * This file was generated automatically
+ * DO NOT MODIFY IT MANUALLY
+ */
+
+interface FirDiagnosticHolder : FirElement {
+    override val source: FirSourceElement?
+    val diagnostic: FirDiagnostic
+
+    override fun <R, D> accept(visitor: FirVisitor<R, D>, data: D): R = visitor.visitDiagnosticHolder(this, data)
+}
diff --git a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/diagnostics/FirSimpleDiagnostic.kt b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/diagnostics/FirSimpleDiagnostic.kt
new file mode 100644
index 00000000000..f572410c6f2
--- /dev/null
+++ b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/diagnostics/FirSimpleDiagnostic.kt
@@ -0,0 +1,8 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.fir.diagnostics
+
+class FirSimpleDiagnostic(override val reason: String) : FirDiagnostic()
\ No newline at end of file
diff --git a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/FirErrorExpression.kt b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/FirErrorExpression.kt
index 0bc428e872e..7db0044f629 100644
--- a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/FirErrorExpression.kt
+++ b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/FirErrorExpression.kt
@@ -7,6 +7,8 @@ package org.jetbrains.kotlin.fir.expressions
 
 import org.jetbrains.kotlin.fir.FirPureAbstractElement
 import org.jetbrains.kotlin.fir.FirSourceElement
+import org.jetbrains.kotlin.fir.diagnostics.FirDiagnostic
+import org.jetbrains.kotlin.fir.diagnostics.FirDiagnosticHolder
 import org.jetbrains.kotlin.fir.types.FirTypeRef
 import org.jetbrains.kotlin.fir.visitors.*
 
@@ -15,11 +17,11 @@ import org.jetbrains.kotlin.fir.visitors.*
  * DO NOT MODIFY IT MANUALLY
  */
 
-abstract class FirErrorExpression : FirPureAbstractElement(), FirExpression {
+abstract class FirErrorExpression : FirPureAbstractElement(), FirExpression, FirDiagnosticHolder {
     abstract override val source: FirSourceElement?
     abstract override val typeRef: FirTypeRef
     abstract override val annotations: List<FirAnnotationCall>
-    abstract val reason: String
+    abstract override val diagnostic: FirDiagnostic
 
     override fun <R, D> accept(visitor: FirVisitor<R, D>, data: D): R = visitor.visitErrorExpression(this, data)
 }
diff --git a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/FirErrorLoop.kt b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/FirErrorLoop.kt
new file mode 100644
index 00000000000..bb00fa4492a
--- /dev/null
+++ b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/FirErrorLoop.kt
@@ -0,0 +1,35 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.fir.expressions
+
+import org.jetbrains.kotlin.fir.FirLabel
+import org.jetbrains.kotlin.fir.FirPureAbstractElement
+import org.jetbrains.kotlin.fir.FirSourceElement
+import org.jetbrains.kotlin.fir.diagnostics.FirDiagnostic
+import org.jetbrains.kotlin.fir.diagnostics.FirDiagnosticHolder
+import org.jetbrains.kotlin.fir.visitors.*
+
+/*
+ * This file was generated automatically
+ * DO NOT MODIFY IT MANUALLY
+ */
+
+abstract class FirErrorLoop : FirPureAbstractElement(), FirLoop, FirDiagnosticHolder {
+    abstract override val source: FirSourceElement?
+    abstract override val annotations: List<FirAnnotationCall>
+    abstract override val block: FirBlock
+    abstract override val condition: FirExpression
+    abstract override val label: FirLabel?
+    abstract override val diagnostic: FirDiagnostic
+
+    override fun <R, D> accept(visitor: FirVisitor<R, D>, data: D): R = visitor.visitErrorLoop(this, data)
+
+    abstract override fun <D> transformBlock(transformer: FirTransformer<D>, data: D): FirErrorLoop
+
+    abstract override fun <D> transformCondition(transformer: FirTransformer<D>, data: D): FirErrorLoop
+
+    abstract override fun <D> transformOtherChildren(transformer: FirTransformer<D>, data: D): FirErrorLoop
+}
diff --git a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/FirExpressionUtil.kt b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/FirExpressionUtil.kt
index c93018b61e9..7616414f9ee 100644
--- a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/FirExpressionUtil.kt
+++ b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/FirExpressionUtil.kt
@@ -6,9 +6,10 @@
 package org.jetbrains.kotlin.fir.expressions
 
 import org.jetbrains.kotlin.fir.FirSourceElement
+import org.jetbrains.kotlin.fir.diagnostics.FirSimpleDiagnostic
 import org.jetbrains.kotlin.fir.expressions.impl.FirConstExpressionImpl
 import org.jetbrains.kotlin.fir.expressions.impl.FirErrorExpressionImpl
-import org.jetbrains.kotlin.fir.expressions.impl.FirErrorLoop
+import org.jetbrains.kotlin.fir.expressions.impl.FirErrorLoopImpl
 import org.jetbrains.kotlin.fir.references.FirResolvedNamedReference
 import org.jetbrains.kotlin.fir.symbols.impl.FirCallableSymbol
 import org.jetbrains.kotlin.fir.types.ConeClassErrorType
@@ -24,7 +25,7 @@ inline val FirAnnotationCall.classId: ClassId?
     get() = coneClassLikeType?.lookupTag?.classId
 
 fun <T> FirConstExpressionImpl(source: FirSourceElement?, kind: IrConstKind<T>, value: T?, errorReason: String): FirExpression =
-    value?.let { FirConstExpressionImpl(source, kind, it) } ?: FirErrorExpressionImpl(source, errorReason)
+    value?.let { FirConstExpressionImpl(source, kind, it) } ?: FirErrorExpressionImpl(source, FirSimpleDiagnostic(errorReason))
 
 inline val FirTypeOperatorCall.argument: FirExpression get() = arguments.first()
 
@@ -36,6 +37,9 @@ fun FirExpression.toResolvedCallableSymbol(): FirCallableSymbol<*>? {
     return toResolvedCallableReference()?.resolvedSymbol as FirCallableSymbol<*>?
 }
 
-fun FirErrorLoop(source: FirSourceElement?, reason: String): FirErrorLoop = FirErrorLoop(source).apply {
-    condition = FirErrorExpressionImpl(source, reason)
+fun FirErrorLoop(source: FirSourceElement?, reason: String): FirErrorLoop {
+    val diagnostic = FirSimpleDiagnostic(reason)
+    return FirErrorLoopImpl(source, diagnostic).apply {
+        condition = FirErrorExpressionImpl(source, diagnostic)
+    }
 }
diff --git a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/impl/FirErrorExpressionImpl.kt b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/impl/FirErrorExpressionImpl.kt
index 18a5a541f6c..caf684cb4d7 100644
--- a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/impl/FirErrorExpressionImpl.kt
+++ b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/impl/FirErrorExpressionImpl.kt
@@ -6,6 +6,7 @@
 package org.jetbrains.kotlin.fir.expressions.impl
 
 import org.jetbrains.kotlin.fir.FirSourceElement
+import org.jetbrains.kotlin.fir.diagnostics.FirDiagnostic
 import org.jetbrains.kotlin.fir.expressions.FirAnnotationCall
 import org.jetbrains.kotlin.fir.expressions.FirErrorExpression
 import org.jetbrains.kotlin.fir.impl.FirAbstractAnnotatedElement
@@ -20,7 +21,7 @@ import org.jetbrains.kotlin.fir.visitors.*
 
 class FirErrorExpressionImpl(
     override val source: FirSourceElement?,
-    override val reason: String
+    override val diagnostic: FirDiagnostic
 ) : FirErrorExpression(), FirAbstractAnnotatedElement {
     override var typeRef: FirTypeRef = FirImplicitTypeRefImpl(null)
     override val annotations: MutableList<FirAnnotationCall> = mutableListOf()
diff --git a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/impl/FirErrorLoop.kt b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/impl/FirErrorLoopImpl.kt
similarity index 81%
rename from compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/impl/FirErrorLoop.kt
rename to compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/impl/FirErrorLoopImpl.kt
index ccff318f28f..53849be4c14 100644
--- a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/impl/FirErrorLoop.kt
+++ b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/impl/FirErrorLoopImpl.kt
@@ -6,12 +6,12 @@
 package org.jetbrains.kotlin.fir.expressions.impl
 
 import org.jetbrains.kotlin.fir.FirLabel
-import org.jetbrains.kotlin.fir.FirPureAbstractElement
 import org.jetbrains.kotlin.fir.FirSourceElement
+import org.jetbrains.kotlin.fir.diagnostics.FirDiagnostic
 import org.jetbrains.kotlin.fir.expressions.FirAnnotationCall
 import org.jetbrains.kotlin.fir.expressions.FirBlock
+import org.jetbrains.kotlin.fir.expressions.FirErrorLoop
 import org.jetbrains.kotlin.fir.expressions.FirExpression
-import org.jetbrains.kotlin.fir.expressions.FirLoop
 import org.jetbrains.kotlin.fir.impl.FirAbstractAnnotatedElement
 import org.jetbrains.kotlin.fir.visitors.*
 
@@ -20,12 +20,13 @@ import org.jetbrains.kotlin.fir.visitors.*
  * DO NOT MODIFY IT MANUALLY
  */
 
-class FirErrorLoop(
-    override val source: FirSourceElement?
-) : FirPureAbstractElement(), FirLoop, FirAbstractAnnotatedElement {
+class FirErrorLoopImpl(
+    override val source: FirSourceElement?,
+    override val diagnostic: FirDiagnostic
+) : FirErrorLoop(), FirAbstractAnnotatedElement {
     override val annotations: MutableList<FirAnnotationCall> = mutableListOf()
     override var block: FirBlock = FirEmptyExpressionBlock()
-    override var condition: FirExpression = FirErrorExpressionImpl(source, "error loop")
+    override var condition: FirExpression = FirErrorExpressionImpl(source, diagnostic)
     override var label: FirLabel? = null
 
     override fun <R, D> acceptChildren(visitor: FirVisitor<R, D>, data: D) {
@@ -35,24 +36,24 @@ class FirErrorLoop(
         label?.accept(visitor, data)
     }
 
-    override fun <D> transformChildren(transformer: FirTransformer<D>, data: D): FirErrorLoop {
+    override fun <D> transformChildren(transformer: FirTransformer<D>, data: D): FirErrorLoopImpl {
         transformBlock(transformer, data)
         transformCondition(transformer, data)
         transformOtherChildren(transformer, data)
         return this
     }
 
-    override fun <D> transformBlock(transformer: FirTransformer<D>, data: D): FirErrorLoop {
+    override fun <D> transformBlock(transformer: FirTransformer<D>, data: D): FirErrorLoopImpl {
         block = block.transformSingle(transformer, data)
         return this
     }
 
-    override fun <D> transformCondition(transformer: FirTransformer<D>, data: D): FirErrorLoop {
+    override fun <D> transformCondition(transformer: FirTransformer<D>, data: D): FirErrorLoopImpl {
         condition = condition.transformSingle(transformer, data)
         return this
     }
 
-    override fun <D> transformOtherChildren(transformer: FirTransformer<D>, data: D): FirErrorLoop {
+    override fun <D> transformOtherChildren(transformer: FirTransformer<D>, data: D): FirErrorLoopImpl {
         annotations.transformInplace(transformer, data)
         label = label?.transformSingle(transformer, data)
         return this
diff --git a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/references/FirErrorNamedReference.kt b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/references/FirErrorNamedReference.kt
index 449b336ee61..d37d2151997 100644
--- a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/references/FirErrorNamedReference.kt
+++ b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/references/FirErrorNamedReference.kt
@@ -6,6 +6,8 @@
 package org.jetbrains.kotlin.fir.references
 
 import org.jetbrains.kotlin.fir.FirSourceElement
+import org.jetbrains.kotlin.fir.diagnostics.FirDiagnostic
+import org.jetbrains.kotlin.fir.diagnostics.FirDiagnosticHolder
 import org.jetbrains.kotlin.fir.symbols.AbstractFirBasedSymbol
 import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.fir.visitors.*
@@ -15,11 +17,11 @@ import org.jetbrains.kotlin.fir.visitors.*
  * DO NOT MODIFY IT MANUALLY
  */
 
-abstract class FirErrorNamedReference : FirNamedReference() {
+abstract class FirErrorNamedReference : FirNamedReference(), FirDiagnosticHolder {
     abstract override val source: FirSourceElement?
     abstract override val name: Name
     abstract override val candidateSymbol: AbstractFirBasedSymbol<*>?
-    abstract val errorReason: String
+    abstract override val diagnostic: FirDiagnostic
 
     override fun <R, D> accept(visitor: FirVisitor<R, D>, data: D): R = visitor.visitErrorNamedReference(this, data)
 }
diff --git a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/references/impl/FirErrorNamedReferenceImpl.kt b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/references/impl/FirErrorNamedReferenceImpl.kt
index 6a9eb423805..165565e6033 100644
--- a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/references/impl/FirErrorNamedReferenceImpl.kt
+++ b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/references/impl/FirErrorNamedReferenceImpl.kt
@@ -6,6 +6,7 @@
 package org.jetbrains.kotlin.fir.references.impl
 
 import org.jetbrains.kotlin.fir.FirSourceElement
+import org.jetbrains.kotlin.fir.diagnostics.FirDiagnostic
 import org.jetbrains.kotlin.fir.references.FirErrorNamedReference
 import org.jetbrains.kotlin.fir.symbols.AbstractFirBasedSymbol
 import org.jetbrains.kotlin.name.Name
@@ -18,9 +19,9 @@ import org.jetbrains.kotlin.fir.visitors.*
 
 class FirErrorNamedReferenceImpl(
     override val source: FirSourceElement?,
-    override val errorReason: String
+    override val diagnostic: FirDiagnostic
 ) : FirErrorNamedReference() {
-    override val name: Name = Name.special("<$errorReason>")
+    override val name: Name = Name.special("<${diagnostic.reason}>")
     override val candidateSymbol: AbstractFirBasedSymbol<*>? get() = null
 
     override fun <R, D> acceptChildren(visitor: FirVisitor<R, D>, data: D) {}
diff --git a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/types/FirErrorTypeRef.kt b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/types/FirErrorTypeRef.kt
index dc7fd47331f..90c3d10b194 100644
--- a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/types/FirErrorTypeRef.kt
+++ b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/types/FirErrorTypeRef.kt
@@ -6,6 +6,8 @@
 package org.jetbrains.kotlin.fir.types
 
 import org.jetbrains.kotlin.fir.FirSourceElement
+import org.jetbrains.kotlin.fir.diagnostics.FirDiagnostic
+import org.jetbrains.kotlin.fir.diagnostics.FirDiagnosticHolder
 import org.jetbrains.kotlin.fir.expressions.FirAnnotationCall
 import org.jetbrains.kotlin.fir.visitors.*
 
@@ -14,11 +16,11 @@ import org.jetbrains.kotlin.fir.visitors.*
  * DO NOT MODIFY IT MANUALLY
  */
 
-abstract class FirErrorTypeRef : FirResolvedTypeRef() {
+abstract class FirErrorTypeRef : FirResolvedTypeRef(), FirDiagnosticHolder {
     abstract override val source: FirSourceElement?
     abstract override val annotations: List<FirAnnotationCall>
     abstract override val type: ConeKotlinType
-    abstract val reason: String
+    abstract override val diagnostic: FirDiagnostic
 
     override fun <R, D> accept(visitor: FirVisitor<R, D>, data: D): R = visitor.visitErrorTypeRef(this, data)
 }
diff --git a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/types/impl/FirErrorTypeRefImpl.kt b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/types/impl/FirErrorTypeRefImpl.kt
index 907bd03eefa..f273d09ca97 100644
--- a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/types/impl/FirErrorTypeRefImpl.kt
+++ b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/types/impl/FirErrorTypeRefImpl.kt
@@ -6,6 +6,7 @@
 package org.jetbrains.kotlin.fir.types.impl
 
 import org.jetbrains.kotlin.fir.FirSourceElement
+import org.jetbrains.kotlin.fir.diagnostics.FirDiagnostic
 import org.jetbrains.kotlin.fir.expressions.FirAnnotationCall
 import org.jetbrains.kotlin.fir.impl.FirAbstractAnnotatedElement
 import org.jetbrains.kotlin.fir.types.ConeClassErrorType
@@ -20,10 +21,10 @@ import org.jetbrains.kotlin.fir.visitors.*
 
 class FirErrorTypeRefImpl(
     override val source: FirSourceElement?,
-    override val reason: String
+    override val diagnostic: FirDiagnostic
 ) : FirErrorTypeRef(), FirAbstractAnnotatedElement {
     override val annotations: MutableList<FirAnnotationCall> = mutableListOf()
-    override val type: ConeKotlinType = ConeClassErrorType(reason)
+    override val type: ConeKotlinType = ConeClassErrorType(diagnostic.reason)
 
     override fun <R, D> acceptChildren(visitor: FirVisitor<R, D>, data: D) {
         annotations.forEach { it.accept(visitor, data) }
diff --git a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/visitors/FirTransformer.kt b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/visitors/FirTransformer.kt
index 0b3b77e5e7e..b4bee265360 100644
--- a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/visitors/FirTransformer.kt
+++ b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/visitors/FirTransformer.kt
@@ -48,7 +48,9 @@ import org.jetbrains.kotlin.fir.declarations.FirConstructor
 import org.jetbrains.kotlin.fir.declarations.FirFile
 import org.jetbrains.kotlin.fir.declarations.FirAnonymousFunction
 import org.jetbrains.kotlin.fir.declarations.FirAnonymousObject
+import org.jetbrains.kotlin.fir.diagnostics.FirDiagnosticHolder
 import org.jetbrains.kotlin.fir.expressions.FirLoop
+import org.jetbrains.kotlin.fir.expressions.FirErrorLoop
 import org.jetbrains.kotlin.fir.expressions.FirDoWhileLoop
 import org.jetbrains.kotlin.fir.expressions.FirWhileLoop
 import org.jetbrains.kotlin.fir.expressions.FirBlock
@@ -294,10 +296,18 @@ abstract class FirTransformer<in D> : FirVisitor<CompositeTransformResult<FirEle
         return transformElement(anonymousObject, data)
     }
 
+    open fun transformDiagnosticHolder(diagnosticHolder: FirDiagnosticHolder, data: D): CompositeTransformResult<FirDiagnosticHolder> {
+        return transformElement(diagnosticHolder, data)
+    }
+
     open fun transformLoop(loop: FirLoop, data: D): CompositeTransformResult<FirStatement> {
         return transformElement(loop, data)
     }
 
+    open fun transformErrorLoop(errorLoop: FirErrorLoop, data: D): CompositeTransformResult<FirStatement> {
+        return transformElement(errorLoop, data)
+    }
+
     open fun transformDoWhileLoop(doWhileLoop: FirDoWhileLoop, data: D): CompositeTransformResult<FirStatement> {
         return transformElement(doWhileLoop, data)
     }
@@ -734,10 +744,18 @@ abstract class FirTransformer<in D> : FirVisitor<CompositeTransformResult<FirEle
         return transformAnonymousObject(anonymousObject, data)
     }
 
+    final override fun visitDiagnosticHolder(diagnosticHolder: FirDiagnosticHolder, data: D): CompositeTransformResult<FirDiagnosticHolder> {
+        return transformDiagnosticHolder(diagnosticHolder, data)
+    }
+
     final override fun visitLoop(loop: FirLoop, data: D): CompositeTransformResult<FirStatement> {
         return transformLoop(loop, data)
     }
 
+    final override fun visitErrorLoop(errorLoop: FirErrorLoop, data: D): CompositeTransformResult<FirStatement> {
+        return transformErrorLoop(errorLoop, data)
+    }
+
     final override fun visitDoWhileLoop(doWhileLoop: FirDoWhileLoop, data: D): CompositeTransformResult<FirStatement> {
         return transformDoWhileLoop(doWhileLoop, data)
     }
diff --git a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/visitors/FirVisitor.kt b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/visitors/FirVisitor.kt
index e0f4ee9bdd2..aa8b490af6e 100644
--- a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/visitors/FirVisitor.kt
+++ b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/visitors/FirVisitor.kt
@@ -48,7 +48,9 @@ import org.jetbrains.kotlin.fir.declarations.FirConstructor
 import org.jetbrains.kotlin.fir.declarations.FirFile
 import org.jetbrains.kotlin.fir.declarations.FirAnonymousFunction
 import org.jetbrains.kotlin.fir.declarations.FirAnonymousObject
+import org.jetbrains.kotlin.fir.diagnostics.FirDiagnosticHolder
 import org.jetbrains.kotlin.fir.expressions.FirLoop
+import org.jetbrains.kotlin.fir.expressions.FirErrorLoop
 import org.jetbrains.kotlin.fir.expressions.FirDoWhileLoop
 import org.jetbrains.kotlin.fir.expressions.FirWhileLoop
 import org.jetbrains.kotlin.fir.expressions.FirBlock
@@ -208,8 +210,12 @@ abstract class FirVisitor<out R, in D> {
 
     open fun visitAnonymousObject(anonymousObject: FirAnonymousObject, data: D): R  = visitElement(anonymousObject, data)
 
+    open fun visitDiagnosticHolder(diagnosticHolder: FirDiagnosticHolder, data: D): R  = visitElement(diagnosticHolder, data)
+
     open fun visitLoop(loop: FirLoop, data: D): R  = visitElement(loop, data)
 
+    open fun visitErrorLoop(errorLoop: FirErrorLoop, data: D): R  = visitElement(errorLoop, data)
+
     open fun visitDoWhileLoop(doWhileLoop: FirDoWhileLoop, data: D): R  = visitElement(doWhileLoop, data)
 
     open fun visitWhileLoop(whileLoop: FirWhileLoop, data: D): R  = visitElement(whileLoop, data)
diff --git a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/visitors/FirVisitorVoid.kt b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/visitors/FirVisitorVoid.kt
index 9181f3151e6..5f1bf39ff55 100644
--- a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/visitors/FirVisitorVoid.kt
+++ b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/visitors/FirVisitorVoid.kt
@@ -48,7 +48,9 @@ import org.jetbrains.kotlin.fir.declarations.FirConstructor
 import org.jetbrains.kotlin.fir.declarations.FirFile
 import org.jetbrains.kotlin.fir.declarations.FirAnonymousFunction
 import org.jetbrains.kotlin.fir.declarations.FirAnonymousObject
+import org.jetbrains.kotlin.fir.diagnostics.FirDiagnosticHolder
 import org.jetbrains.kotlin.fir.expressions.FirLoop
+import org.jetbrains.kotlin.fir.expressions.FirErrorLoop
 import org.jetbrains.kotlin.fir.expressions.FirDoWhileLoop
 import org.jetbrains.kotlin.fir.expressions.FirWhileLoop
 import org.jetbrains.kotlin.fir.expressions.FirBlock
@@ -292,10 +294,18 @@ abstract class FirVisitorVoid : FirVisitor<Unit, Nothing?>() {
         visitElement(anonymousObject)
     }
 
+    open fun visitDiagnosticHolder(diagnosticHolder: FirDiagnosticHolder) {
+        visitElement(diagnosticHolder)
+    }
+
     open fun visitLoop(loop: FirLoop) {
         visitElement(loop)
     }
 
+    open fun visitErrorLoop(errorLoop: FirErrorLoop) {
+        visitElement(errorLoop)
+    }
+
     open fun visitDoWhileLoop(doWhileLoop: FirDoWhileLoop) {
         visitElement(doWhileLoop)
     }
@@ -732,10 +742,18 @@ abstract class FirVisitorVoid : FirVisitor<Unit, Nothing?>() {
         visitAnonymousObject(anonymousObject)
     }
 
+    final override fun visitDiagnosticHolder(diagnosticHolder: FirDiagnosticHolder, data: Nothing?) {
+        visitDiagnosticHolder(diagnosticHolder)
+    }
+
     final override fun visitLoop(loop: FirLoop, data: Nothing?) {
         visitLoop(loop)
     }
 
+    final override fun visitErrorLoop(errorLoop: FirErrorLoop, data: Nothing?) {
+        visitErrorLoop(errorLoop)
+    }
+
     final override fun visitDoWhileLoop(doWhileLoop: FirDoWhileLoop, data: Nothing?) {
         visitDoWhileLoop(doWhileLoop)
     }
diff --git a/compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/FirTreeBuilder.kt b/compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/FirTreeBuilder.kt
index f3cd27fcbbc..68643859ef3 100644
--- a/compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/FirTreeBuilder.kt
+++ b/compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/FirTreeBuilder.kt
@@ -59,7 +59,10 @@ object FirTreeBuilder : AbstractFirTreeBuilder() {
     val anonymousFunction = element("AnonymousFunction", Declaration, function, expression)
     val anonymousObject = element("AnonymousObject", Declaration, klass, expression)
 
+    val diagnosticHolder = element("DiagnosticHolder", Diagnostics)
+
     val loop = element("Loop", Expression, statement, targetElement, annotationContainer)
+    val errorLoop = element("ErrorLoop", Expression, loop, diagnosticHolder)
     val doWhileLoop = element("DoWhileLoop", Expression, loop)
     val whileLoop = element("WhileLoop", Expression, loop)
 
@@ -88,8 +91,8 @@ object FirTreeBuilder : AbstractFirTreeBuilder() {
     val arrayOfCall = element("ArrayOfCall", Expression, expression, call)
     val arraySetCall = element("ArraySetCall", Expression, qualifiedAccess, call)
     val classReferenceExpression = element("ClassReferenceExpression", Expression, expression)
-    val errorExpression = element("ErrorExpression", Expression, expression)
-    val errorFunction = element("ErrorFunction", Declaration, function)
+    val errorExpression = element("ErrorExpression", Expression, expression, diagnosticHolder)
+    val errorFunction = element("ErrorFunction", Declaration, function, diagnosticHolder)
     val qualifiedAccessExpression = element("QualifiedAccessExpression", Expression, expression, qualifiedAccess)
     val functionCall = element("FunctionCall", Expression, qualifiedAccessExpression, call)
     val componentCall = element("ComponentCall", Expression, functionCall)
@@ -113,7 +116,7 @@ object FirTreeBuilder : AbstractFirTreeBuilder() {
     val wrappedDelegateExpression = element("WrappedDelegateExpression", Expression, wrappedExpression)
 
     val namedReference = element("NamedReference", Reference, reference)
-    val errorNamedReference = element("ErrorNamedReference", Reference, namedReference)
+    val errorNamedReference = element("ErrorNamedReference", Reference, namedReference, diagnosticHolder)
     val superReference = element("SuperReference", Reference, reference)
     val thisReference = element("ThisReference", Reference, reference)
     val controlFlowGraphReference = element("ControlFlowGraphReference", Reference, reference)
@@ -125,7 +128,7 @@ object FirTreeBuilder : AbstractFirTreeBuilder() {
     val resolvedCallableReference = element("ResolvedCallableReference", Reference, resolvedNamedReference)
 
     val resolvedTypeRef = element("ResolvedTypeRef", TypeRef, typeRef)
-    val errorTypeRef = element("ErrorTypeRef", TypeRef, resolvedTypeRef)
+    val errorTypeRef = element("ErrorTypeRef", TypeRef, resolvedTypeRef, diagnosticHolder)
     val delegatedTypeRef = element("DelegatedTypeRef", TypeRef, typeRef)
     val typeRefWithNullability = element("TypeRefWithNullability", TypeRef, typeRef)
     val userTypeRef = element("UserTypeRef", TypeRef, typeRefWithNullability)
diff --git a/compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/ImplementationConfigurator.kt b/compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/ImplementationConfigurator.kt
index 2a4483f108a..224adfacec1 100644
--- a/compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/ImplementationConfigurator.kt
+++ b/compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/ImplementationConfigurator.kt
@@ -224,11 +224,9 @@ object ImplementationConfigurator : AbstractFirTreeImplementationConfigurator()
             noSource()
         }
 
-        impl(errorExpression)
-
-        impl(loop, "FirErrorLoop") {
+        impl(errorLoop) {
             default("block", "FirEmptyExpressionBlock()")
-            default("condition", "FirErrorExpressionImpl(source, \"error loop\")")
+            default("condition", "FirErrorExpressionImpl(source, diagnostic)")
             defaultNull("label")
             useTypes(emptyExpressionBlock)
         }
@@ -496,13 +494,13 @@ object ImplementationConfigurator : AbstractFirTreeImplementationConfigurator()
         impl(resolvedTypeRef)
 
         val errorTypeRefImpl = impl(errorTypeRef) {
-            default("type", "ConeClassErrorType(reason)")
+            default("type", "ConeClassErrorType(diagnostic.reason)")
             useTypes(coneClassErrorTypeType)
         }
 
         impl(errorFunction) {
             defaultNull("receiverTypeRef", "body", withGetter = true)
-            default("returnTypeRef", "FirErrorTypeRefImpl(null, reason)")
+            default("returnTypeRef", "FirErrorTypeRefImpl(null, diagnostic)")
             useTypes(errorTypeRefImpl)
         }
 
@@ -524,7 +522,7 @@ object ImplementationConfigurator : AbstractFirTreeImplementationConfigurator()
         }
 
         impl(errorNamedReference) {
-            default("name", "Name.special(\"<\$errorReason>\")")
+            default("name", "Name.special(\"<\$diagnostic.reason>\")")
             defaultNull("candidateSymbol", withGetter = true)
         }
 
diff --git a/compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/NodeConfigurator.kt b/compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/NodeConfigurator.kt
index bc002276629..0c503dacece 100644
--- a/compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/NodeConfigurator.kt
+++ b/compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/NodeConfigurator.kt
@@ -59,6 +59,10 @@ object NodeConfigurator : AbstractFieldConfigurator() {
             +field("calleeReference", reference).withTransform()
         }
 
+        diagnosticHolder.configure {
+            +field("diagnostic", firDiagnosticType)
+        }
+
         declaration.configure {
             +field("session", firSessionType)
             +field("resolvePhase", resolvePhaseType, withReplace = true).apply { isMutable = true }
@@ -95,7 +99,6 @@ object NodeConfigurator : AbstractFieldConfigurator() {
 
         errorFunction.configure {
             parentArg(function, "F", errorFunction)
-            +stringField("reason")
             +symbol("FirErrorFunctionSymbol")
         }
 
@@ -398,10 +401,6 @@ object NodeConfigurator : AbstractFieldConfigurator() {
             +intField("componentIndex")
         }
 
-        errorExpression.configure {
-            +stringField("reason")
-        }
-
         expressionWithSmartcast.configure {
             +field("originalExpression", qualifiedAccessExpression)
             +field("typesFromSmartcast", "Collection<ConeKotlinType>", null, customType = coneKotlinTypeType)
@@ -493,10 +492,6 @@ object NodeConfigurator : AbstractFieldConfigurator() {
             +field("type", coneKotlinTypeType)
         }
 
-        errorTypeRef.configure {
-            +stringField("reason")
-        }
-
         delegatedTypeRef.configure {
             +field("delegate", expression, nullable = true)
             +field(typeRef)
@@ -533,10 +528,6 @@ object NodeConfigurator : AbstractFieldConfigurator() {
             +field(varianceType)
         }
 
-        errorNamedReference.configure {
-            +stringField("errorReason")
-        }
-
         contractDescription.configure {
             +fieldList("effects", effectDeclarationType)
         }
diff --git a/compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/Types.kt b/compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/Types.kt
index 34f45b81b21..0cd27a7215e 100644
--- a/compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/Types.kt
+++ b/compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/Types.kt
@@ -62,4 +62,5 @@ val classSymbolType = type("fir.symbols.impl", "FirClassSymbol")
 
 val pureAbstractElementType = generatedType("FirPureAbstractElement")
 val effectDeclarationType = type("fir.contracts.description", "ConeEffectDeclaration")
-val emptyContractDescriptionType = generatedType("contracts.impl", "FirEmptyContractDescription")
\ No newline at end of file
+val emptyContractDescriptionType = generatedType("contracts.impl", "FirEmptyContractDescription")
+val firDiagnosticType = generatedType("diagnostics", "FirDiagnostic")
\ No newline at end of file
diff --git a/compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/model/Element.kt b/compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/model/Element.kt
index 7645c085d88..e149039de05 100644
--- a/compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/model/Element.kt
+++ b/compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/model/Element.kt
@@ -125,6 +125,7 @@ class Element(val name: String, kind: Kind) : AbstractElement {
         Reference("references"),
         TypeRef("types"),
         Contracts("contracts"),
+        Diagnostics("diagnostics"),
         Other("")
     }
 }
diff --git a/compiler/visualizer/render-fir/src/org/jetbrains/kotlin/compiler/visualizer/FirVisualizer.kt b/compiler/visualizer/render-fir/src/org/jetbrains/kotlin/compiler/visualizer/FirVisualizer.kt
index bf71f3d0d57..fcf96934829 100644
--- a/compiler/visualizer/render-fir/src/org/jetbrains/kotlin/compiler/visualizer/FirVisualizer.kt
+++ b/compiler/visualizer/render-fir/src/org/jetbrains/kotlin/compiler/visualizer/FirVisualizer.kt
@@ -358,7 +358,7 @@ class FirVisualizer(private val firFile: FirFile) : BaseRenderer() {
 
         override fun visitNamedReference(namedReference: FirNamedReference, data: StringBuilder) {
             if (namedReference is FirErrorNamedReference) {
-                data.append("[ERROR : ${namedReference.errorReason}]")
+                data.append("[ERROR : ${namedReference.diagnostic.reason}]")
                 return
             }
             visitElement(namedReference, data)
@@ -416,7 +416,7 @@ class FirVisualizer(private val firFile: FirFile) : BaseRenderer() {
                         functionCall.typeRef.accept(this, data)
                     }
                 }
-                is FirErrorNamedReference -> data.append("[ERROR : ${callee.errorReason}]")
+                is FirErrorNamedReference -> data.append("[ERROR : ${callee.diagnostic.reason}]")
             }
         }
 
@@ -470,7 +470,7 @@ class FirVisualizer(private val firFile: FirFile) : BaseRenderer() {
         }
 
         override fun visitErrorTypeRef(errorTypeRef: FirErrorTypeRef, data: StringBuilder) {
-            data.append("[ERROR : ${errorTypeRef.reason}]")
+            data.append("[ERROR : ${errorTypeRef.diagnostic.reason}]")
         }
 
         override fun visitResolvedFunctionTypeRef(resolvedFunctionTypeRef: FirResolvedFunctionTypeRef, data: StringBuilder) {