diff --git a/compiler/fir/analysis-tests/testData/resolve/diagnostics/methodOfAnyImplementedInInterface.kt b/compiler/fir/analysis-tests/testData/resolve/diagnostics/methodOfAnyImplementedInInterface.kt
index a13c1084fb8..0c79a69827f 100644
--- a/compiler/fir/analysis-tests/testData/resolve/diagnostics/methodOfAnyImplementedInInterface.kt
+++ b/compiler/fir/analysis-tests/testData/resolve/diagnostics/methodOfAnyImplementedInInterface.kt
@@ -13,13 +13,13 @@ interface B {
 }
 
 interface C {
-    <!METHOD_OF_ANY_IMPLEMENTED_IN_INTERFACE!>override operator fun toString(): String = "Rest"<!>
+    <!METHOD_OF_ANY_IMPLEMENTED_IN_INTERFACE!>override <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun toString(): String = "Rest"<!>
     <!METHOD_OF_ANY_IMPLEMENTED_IN_INTERFACE!>override operator fun equals(other: Any?): Boolean = false<!>
-    <!METHOD_OF_ANY_IMPLEMENTED_IN_INTERFACE!>override operator fun hashCode(): Int = 2<!>
+    <!METHOD_OF_ANY_IMPLEMENTED_IN_INTERFACE!>override <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun hashCode(): Int = 2<!>
 }
 
 interface D {
-    override operator fun toString(): String
+    override <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun toString(): String
     override operator fun equals(other: Any?): Boolean
-    override operator fun hashCode(): Int
+    override <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun hashCode(): Int
 }
diff --git a/compiler/fir/analysis-tests/testData/resolveWithStdlib/diagnostics/forLoopChecker.kt b/compiler/fir/analysis-tests/testData/resolveWithStdlib/diagnostics/forLoopChecker.kt
index 658e51e708a..17e882268ef 100644
--- a/compiler/fir/analysis-tests/testData/resolveWithStdlib/diagnostics/forLoopChecker.kt
+++ b/compiler/fir/analysis-tests/testData/resolveWithStdlib/diagnostics/forLoopChecker.kt
@@ -25,7 +25,7 @@ abstract class NotRange4() {
 }
 
 abstract class ImproperIterator3 {
-    abstract operator fun hasNext() : Int
+    abstract <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun hasNext() : Int
     abstract operator fun next() : Int
 }
 
diff --git a/compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/FirDiagnosticsList.kt b/compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/FirDiagnosticsList.kt
index ebb8637d3ee..fd82f8b4fe7 100644
--- a/compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/FirDiagnosticsList.kt
+++ b/compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/FirDiagnosticsList.kt
@@ -347,6 +347,9 @@ object DIAGNOSTICS_LIST : DiagnosticList("FirErrors") {
             parameter<KtModifierKeywordToken>("modifier")
             parameter<String>("target")
         }
+        val INAPPLICABLE_OPERATOR_MODIFIER by error<PsiElement>(PositioningStrategy.OPERATOR_MODIFIER) {
+            parameter<String>("message")
+        }
     }
 
     val INLINE_CLASSES by object : DiagnosticGroup("Inline classes") {
diff --git a/compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/model/RegularDiagnosticData.kt b/compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/model/RegularDiagnosticData.kt
index 533bf940dc2..38c55759ecd 100644
--- a/compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/model/RegularDiagnosticData.kt
+++ b/compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/model/RegularDiagnosticData.kt
@@ -103,6 +103,7 @@ enum class PositioningStrategy(private val strategy: String? = null) {
     ABSTRACT_MODIFIER,
     LABEL,
     COMMAS,
+    OPERATOR_MODIFIER,
 
     ;
 
diff --git a/compiler/fir/checkers/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/FirErrors.kt b/compiler/fir/checkers/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/FirErrors.kt
index 12ac8e60291..c390c681338 100644
--- a/compiler/fir/checkers/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/FirErrors.kt
+++ b/compiler/fir/checkers/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/FirErrors.kt
@@ -268,6 +268,7 @@ object FirErrors {
     val INFIX_MODIFIER_REQUIRED by error1<PsiElement, FirNamedFunctionSymbol>()
     val WRONG_MODIFIER_CONTAINING_DECLARATION by error2<PsiElement, KtModifierKeywordToken, String>()
     val DEPRECATED_MODIFIER_CONTAINING_DECLARATION by warning2<PsiElement, KtModifierKeywordToken, String>()
+    val INAPPLICABLE_OPERATOR_MODIFIER by error1<PsiElement, String>(SourceElementPositioningStrategies.OPERATOR_MODIFIER)
 
     // Inline classes
     val INLINE_CLASS_NOT_TOP_LEVEL by error0<KtDeclaration>(SourceElementPositioningStrategies.INLINE_OR_VALUE_MODIFIER)
diff --git a/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/CommonDeclarationCheckers.kt b/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/CommonDeclarationCheckers.kt
index c93fc419929..7dc3cbcc99c 100644
--- a/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/CommonDeclarationCheckers.kt
+++ b/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/CommonDeclarationCheckers.kt
@@ -45,6 +45,7 @@ object CommonDeclarationCheckers : DeclarationCheckers() {
         get() = setOf(
             FirFunctionNameChecker,
             FirFunctionTypeParametersSyntaxChecker,
+            FirOperatorModifierChecker,
         )
 
     override val propertyCheckers: Set<FirPropertyChecker>
diff --git a/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/declaration/FirOperatorModifierChecker.kt b/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/declaration/FirOperatorModifierChecker.kt
new file mode 100644
index 00000000000..50686eec512
--- /dev/null
+++ b/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/declaration/FirOperatorModifierChecker.kt
@@ -0,0 +1,221 @@
+/*
+ * Copyright 2010-2021 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.
+ */
+
+/*
+ * Copyright 2010-2021 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.analysis.checkers.declaration
+
+import javaslang.Function2
+import org.jetbrains.kotlin.builtins.StandardNames
+import org.jetbrains.kotlin.fir.analysis.checkers.context.CheckerContext
+import org.jetbrains.kotlin.fir.analysis.checkers.declaration.Checks.Returns
+import org.jetbrains.kotlin.fir.analysis.checkers.declaration.Checks.ValueParametersCount
+import org.jetbrains.kotlin.fir.analysis.checkers.declaration.Checks.isKProperty
+import org.jetbrains.kotlin.fir.analysis.checkers.declaration.Checks.member
+import org.jetbrains.kotlin.fir.analysis.checkers.declaration.Checks.memberOrExtension
+import org.jetbrains.kotlin.fir.analysis.checkers.declaration.Checks.noDefaultAndVarargs
+import org.jetbrains.kotlin.fir.analysis.checkers.hasModifier
+import org.jetbrains.kotlin.fir.analysis.checkers.isSubtypeOf
+import org.jetbrains.kotlin.fir.analysis.checkers.isSupertypeOf
+import org.jetbrains.kotlin.fir.analysis.checkers.overriddenFunctions
+import org.jetbrains.kotlin.fir.analysis.diagnostics.DiagnosticReporter
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors
+import org.jetbrains.kotlin.fir.analysis.diagnostics.reportOn
+import org.jetbrains.kotlin.fir.containingClass
+import org.jetbrains.kotlin.fir.declarations.FirSimpleFunction
+import org.jetbrains.kotlin.fir.declarations.utils.isOperator
+import org.jetbrains.kotlin.fir.resolve.toFirRegularClass
+import org.jetbrains.kotlin.fir.symbols.impl.ConeClassLikeLookupTagImpl
+import org.jetbrains.kotlin.fir.typeContext
+import org.jetbrains.kotlin.fir.types.*
+import org.jetbrains.kotlin.fir.types.impl.ConeClassLikeTypeImpl
+import org.jetbrains.kotlin.lexer.KtTokens
+import org.jetbrains.kotlin.name.Name
+import org.jetbrains.kotlin.util.OperatorNameConventions.ASSIGNMENT_OPERATIONS
+import org.jetbrains.kotlin.util.OperatorNameConventions.BINARY_OPERATION_NAMES
+import org.jetbrains.kotlin.util.OperatorNameConventions.COMPARE_TO
+import org.jetbrains.kotlin.util.OperatorNameConventions.COMPONENT_REGEX
+import org.jetbrains.kotlin.util.OperatorNameConventions.CONTAINS
+import org.jetbrains.kotlin.util.OperatorNameConventions.DEC
+import org.jetbrains.kotlin.util.OperatorNameConventions.EQUALS
+import org.jetbrains.kotlin.util.OperatorNameConventions.GET
+import org.jetbrains.kotlin.util.OperatorNameConventions.GET_VALUE
+import org.jetbrains.kotlin.util.OperatorNameConventions.HAS_NEXT
+import org.jetbrains.kotlin.util.OperatorNameConventions.INC
+import org.jetbrains.kotlin.util.OperatorNameConventions.INVOKE
+import org.jetbrains.kotlin.util.OperatorNameConventions.ITERATOR
+import org.jetbrains.kotlin.util.OperatorNameConventions.NEXT
+import org.jetbrains.kotlin.util.OperatorNameConventions.PROVIDE_DELEGATE
+import org.jetbrains.kotlin.util.OperatorNameConventions.RANGE_TO
+import org.jetbrains.kotlin.util.OperatorNameConventions.SET
+import org.jetbrains.kotlin.util.OperatorNameConventions.SET_VALUE
+import org.jetbrains.kotlin.util.OperatorNameConventions.SIMPLE_UNARY_OPERATION_NAMES
+
+
+object FirOperatorModifierChecker : FirSimpleFunctionChecker() {
+
+    override fun check(declaration: FirSimpleFunction, context: CheckerContext, reporter: DiagnosticReporter) {
+        if (!declaration.isOperator) return
+        //we are not interested in implicit operators from override
+        if (!declaration.hasModifier(KtTokens.OPERATOR_KEYWORD)) return
+        
+        val checks = OperatorFunctionChecks.checksByName.getOrElse(declaration.name) {
+            OperatorFunctionChecks.regexChecks.find { it.first.matches(declaration.name.asString()) }?.second
+        }
+
+        if (checks == null) {
+            reporter.reportOn(declaration.source, FirErrors.INAPPLICABLE_OPERATOR_MODIFIER, "illegal function name", context)
+            return
+        }
+
+        for (check in checks) {
+            check.check(context, declaration)?.let { error ->
+                reporter.reportOn(declaration.source, FirErrors.INAPPLICABLE_OPERATOR_MODIFIER, error, context)
+                return
+            }
+        }
+    }
+
+}
+
+interface Check : Function2<CheckerContext, FirSimpleFunction, String?> {
+    override fun apply(t1: CheckerContext, t2: FirSimpleFunction): String? = check(t1, t2)
+    fun check(context: CheckerContext, function: FirSimpleFunction): String?
+}
+
+object Checks {
+    fun simple(message: String, predicate: (FirSimpleFunction) -> Boolean) = object : Check {
+        override fun check(context: CheckerContext, function: FirSimpleFunction): String? = message.takeIf { !predicate(function) }
+    }
+
+    fun full(message: String, predicate: (CheckerContext, FirSimpleFunction) -> Boolean) = object : Check {
+        override fun check(context: CheckerContext, function: FirSimpleFunction): String? = message.takeIf { !predicate(context, function) }
+    }
+
+    val memberOrExtension = simple("must be a member or an extension function") {
+        it.dispatchReceiverType != null || it.receiverTypeRef != null
+    }
+
+    val member = simple("must be a member function") {
+        it.dispatchReceiverType != null
+    }
+
+    object ValueParametersCount {
+        fun atLeast(n: Int) = simple("must have at least $n value parameter" + (if (n > 1) "s" else "")) {
+            it.valueParameters.size >= n
+        }
+
+        fun exactly(n: Int) = simple("must have exactly $n value parameters") {
+            it.valueParameters.size == n
+        }
+
+        val single = simple("must have a single value parameter") {
+            it.valueParameters.size == 1
+        }
+        val none = simple("must have no value parameters") {
+            it.valueParameters.isEmpty()
+        }
+    }
+
+    object Returns {
+        val boolean = simple("must return Boolean") {
+            it.returnTypeRef.isBoolean
+        }
+
+        val int = simple("must return Int") {
+            it.returnTypeRef.isInt
+        }
+
+        val unit = simple("must return Unit") {
+            it.returnTypeRef.isUnit
+        }
+    }
+
+    val noDefaultAndVarargs = simple("should not have varargs or parameters with default values") {
+        it.valueParameters.all { param ->
+            param.defaultValue == null && !param.isVararg
+        }
+    }
+
+    private val kPropertyType = ConeClassLikeTypeImpl(
+        ConeClassLikeLookupTagImpl(StandardNames.FqNames.kProperty),
+        arrayOf(ConeStarProjection),
+        isNullable = false
+    )
+
+    val isKProperty = full("second parameter must be of type KProperty<*> or its supertype") { ctx, function ->
+        val paramType = function.valueParameters[1].returnTypeRef.coneType
+        paramType.isSupertypeOf(ctx.session.typeContext, kPropertyType)
+    }
+
+}
+
+@OptIn(ExperimentalStdlibApi::class)
+object OperatorFunctionChecks {
+
+    //reimplementation of org.jetbrains.kotlin.util.OperatorChecks for FIR
+    val checksByName: Map<Name, List<Check>> = buildMap<Name, List<Check>> {
+        checkFor(GET, memberOrExtension, ValueParametersCount.atLeast(1))
+        checkFor(
+            SET,
+            memberOrExtension, ValueParametersCount.atLeast(2),
+            Checks.simple("last parameter should not have a default value or be a vararg") {
+                it.valueParameters.lastOrNull()?.let { param ->
+                    param.defaultValue == null && !param.isVararg
+                } == true
+            }
+        )
+        checkFor(GET_VALUE, memberOrExtension, noDefaultAndVarargs, ValueParametersCount.atLeast(2), isKProperty)
+        checkFor(SET_VALUE, memberOrExtension, noDefaultAndVarargs, ValueParametersCount.atLeast(3), isKProperty)
+        checkFor(PROVIDE_DELEGATE, memberOrExtension, noDefaultAndVarargs, ValueParametersCount.exactly(2), isKProperty)
+        checkFor(INVOKE, memberOrExtension)
+        checkFor(CONTAINS, memberOrExtension, ValueParametersCount.single, noDefaultAndVarargs, Returns.boolean)
+        checkFor(ITERATOR, memberOrExtension, ValueParametersCount.none)
+        checkFor(NEXT, memberOrExtension, ValueParametersCount.none)
+        checkFor(HAS_NEXT, memberOrExtension, ValueParametersCount.none, Returns.boolean)
+        checkFor(RANGE_TO, memberOrExtension, ValueParametersCount.single, noDefaultAndVarargs)
+        checkFor(
+            EQUALS,
+            member,
+            Checks.full("must override ''equals()'' in Any") { ctx, function ->
+                val containingClass = function.containingClass()?.toFirRegularClass(ctx.session) ?: return@full true
+                function.overriddenFunctions(containingClass, ctx).any {
+                    it.containingClass()?.classId?.asSingleFqName() == StandardNames.FqNames.any.toSafe()
+                }
+            }
+        )
+        checkFor(COMPARE_TO, memberOrExtension, Returns.int, ValueParametersCount.single, noDefaultAndVarargs)
+        checkFor(BINARY_OPERATION_NAMES, memberOrExtension, ValueParametersCount.single, noDefaultAndVarargs)
+        checkFor(SIMPLE_UNARY_OPERATION_NAMES, memberOrExtension, ValueParametersCount.none)
+        checkFor(
+            setOf(INC, DEC),
+            memberOrExtension,
+            Checks.full("receiver must be a supertype of the return type") { ctx, function ->
+                val receiver = function.dispatchReceiverType ?: function.receiverTypeRef?.coneType ?: return@full false
+                function.returnTypeRef.coneType.isSubtypeOf(ctx.session.typeContext, receiver)
+            }
+        )
+        checkFor(ASSIGNMENT_OPERATIONS, memberOrExtension, Returns.unit, ValueParametersCount.single, noDefaultAndVarargs)
+    }
+
+    val regexChecks: List<Pair<Regex, List<Check>>> = buildList {
+        checkFor(COMPONENT_REGEX, memberOrExtension, ValueParametersCount.none)
+    }
+
+    private fun MutableMap<Name, List<Check>>.checkFor(name: Name, vararg checks: Check) {
+        put(name, checks.asList())
+    }
+
+    private fun MutableMap<Name, List<Check>>.checkFor(names: Set<Name>, vararg checks: Check) {
+        names.forEach { put(it, checks.asList()) }
+    }
+
+    private fun MutableList<Pair<Regex, List<Check>>>.checkFor(regex: Regex, vararg checks: Check) {
+        add(regex to checks.asList())
+    }
+}
\ No newline at end of file
diff --git a/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirDefaultErrorMessages.kt b/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirDefaultErrorMessages.kt
index 43356212e2c..e461c5dc848 100644
--- a/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirDefaultErrorMessages.kt
+++ b/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirDefaultErrorMessages.kt
@@ -211,6 +211,7 @@ import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INAPPLICABLE_CAND
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INAPPLICABLE_FILE_TARGET
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INAPPLICABLE_INFIX_MODIFIER
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INAPPLICABLE_LATEINIT_MODIFIER
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INAPPLICABLE_OPERATOR_MODIFIER
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INAPPLICABLE_PARAM_TARGET
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INAPPLICABLE_TARGET_ON_PROPERTY
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INAPPLICABLE_TARGET_PROPERTY_HAS_NO_BACKING_FIELD
@@ -744,6 +745,7 @@ class FirDefaultErrorMessages {
             map.put(INFIX_MODIFIER_REQUIRED, "''infix'' modifier is required on ''{0}''", TO_STRING)
             map.put(WRONG_MODIFIER_CONTAINING_DECLARATION, "Modifier ''{0}'' is not applicable inside ''{1}''", TO_STRING, STRING)
             map.put(DEPRECATED_MODIFIER_CONTAINING_DECLARATION, "Modifier ''{0}'' is deprecated inside ''{1}''", TO_STRING, STRING)
+            map.put(INAPPLICABLE_OPERATOR_MODIFIER, "''operator'' modifier is inapplicable on this function: {0}", STRING)
 
             // Classes and interfaces
             map.put(SUPERTYPE_NOT_INITIALIZED, "This type has a constructor, and thus must be initialized here")
diff --git a/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/LightTreePositioningStrategies.kt b/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/LightTreePositioningStrategies.kt
index 1a3728ab3d7..40d9afd0ed7 100644
--- a/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/LightTreePositioningStrategies.kt
+++ b/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/LightTreePositioningStrategies.kt
@@ -373,6 +373,9 @@ object LightTreePositioningStrategies {
     val DATA_MODIFIER: LightTreePositioningStrategy =
         ModifierSetBasedLightTreePositioningStrategy(TokenSet.create(KtTokens.DATA_KEYWORD))
 
+    val OPERATOR_MODIFIER: LightTreePositioningStrategy =
+        ModifierSetBasedLightTreePositioningStrategy(TokenSet.create(KtTokens.OPERATOR_KEYWORD))
+
     val INLINE_PARAMETER_MODIFIER: LightTreePositioningStrategy =
         ModifierSetBasedLightTreePositioningStrategy(TokenSet.create(KtTokens.NOINLINE_KEYWORD, KtTokens.CROSSINLINE_KEYWORD))
 
diff --git a/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/SourceElementPositioningStrategies.kt b/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/SourceElementPositioningStrategies.kt
index 633eeab63ac..8dca98b8065 100644
--- a/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/SourceElementPositioningStrategies.kt
+++ b/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/SourceElementPositioningStrategies.kt
@@ -304,4 +304,9 @@ object SourceElementPositioningStrategies {
         LightTreePositioningStrategies.INLINE_PARAMETER_MODIFIER,
         PositioningStrategies.INLINE_PARAMETER_MODIFIER
     )
+
+    val OPERATOR_MODIFIER = SourceElementPositioningStrategy(
+        LightTreePositioningStrategies.OPERATOR_MODIFIER,
+        PositioningStrategies.OPERATOR_MODIFIER
+    )
 }
diff --git a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/types/FirTypeUtils.kt b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/types/FirTypeUtils.kt
index 8657e05dc19..f45d2aba5d7 100644
--- a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/types/FirTypeUtils.kt
+++ b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/types/FirTypeUtils.kt
@@ -40,6 +40,7 @@ val FirTypeRef.isNothing: Boolean get() = isBuiltinType(StandardClassIds.Nothing
 val FirTypeRef.isNullableNothing: Boolean get() = isBuiltinType(StandardClassIds.Nothing, true)
 val FirTypeRef.isUnit: Boolean get() = isBuiltinType(StandardClassIds.Unit, false)
 val FirTypeRef.isBoolean: Boolean get() = isBuiltinType(StandardClassIds.Boolean, false)
+val FirTypeRef.isInt: Boolean get() = isBuiltinType(StandardClassIds.Int, false)
 val FirTypeRef.isEnum: Boolean get() = isBuiltinType(StandardClassIds.Enum, false)
 val FirTypeRef.isArrayType: Boolean
     get() =
diff --git a/compiler/frontend/src/org/jetbrains/kotlin/diagnostics/PositioningStrategies.kt b/compiler/frontend/src/org/jetbrains/kotlin/diagnostics/PositioningStrategies.kt
index c162c7d1518..798f208ff35 100644
--- a/compiler/frontend/src/org/jetbrains/kotlin/diagnostics/PositioningStrategies.kt
+++ b/compiler/frontend/src/org/jetbrains/kotlin/diagnostics/PositioningStrategies.kt
@@ -377,6 +377,9 @@ object PositioningStrategies {
     @JvmField
     val DATA_MODIFIER: PositioningStrategy<KtModifierListOwner> = modifierSetPosition(KtTokens.DATA_KEYWORD)
 
+    @JvmField
+    val OPERATOR_MODIFIER: PositioningStrategy<KtModifierListOwner> = modifierSetPosition(KtTokens.OPERATOR_KEYWORD)
+
     @JvmField
     val FOR_REDECLARATION: PositioningStrategy<PsiElement> = object : PositioningStrategy<PsiElement>() {
         override fun mark(element: PsiElement): List<TextRange> {
diff --git a/compiler/testData/diagnostics/tests/DeprecatedUnaryOperatorConventions.fir.kt b/compiler/testData/diagnostics/tests/DeprecatedUnaryOperatorConventions.fir.kt
index eb8df0587ce..c7df06f542f 100644
--- a/compiler/testData/diagnostics/tests/DeprecatedUnaryOperatorConventions.fir.kt
+++ b/compiler/testData/diagnostics/tests/DeprecatedUnaryOperatorConventions.fir.kt
@@ -1,15 +1,15 @@
 // !DIAGNOSTICS: -UNUSED_PARAMETER
 
 class Example {
-    operator fun plus(): String = ""
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun plus(): String = ""
     operator fun unaryPlus(): Int = 0
 }
 
 class ExampleDeprecated {
-    operator fun plus(): String = ""
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun plus(): String = ""
 }
 
-operator fun String.plus(): String = this
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun String.plus(): String = this
 operator fun String.unaryPlus(): Int = 0
 
 fun test() {
@@ -22,8 +22,8 @@ fun requireInt(n: Int) {}
 fun requireString(s: String) {}
 
 class Example2 {
-    operator fun plus() = this
-    operator fun minus() = this
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun plus() = this
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun minus() = this
 
     fun test() {
         <!INAPPLICABLE_CANDIDATE!>+<!>this
diff --git a/compiler/testData/diagnostics/tests/ForRangeConventions.fir.kt b/compiler/testData/diagnostics/tests/ForRangeConventions.fir.kt
index 4ce159082d3..f7fb90cfb46 100644
--- a/compiler/testData/diagnostics/tests/ForRangeConventions.fir.kt
+++ b/compiler/testData/diagnostics/tests/ForRangeConventions.fir.kt
@@ -27,7 +27,7 @@ abstract class NotRange4() {
 }
 
 abstract class ImproperIterator3 {
-  abstract operator fun hasNext() : Int
+  abstract <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun hasNext() : Int
   abstract operator fun next() : Int
 }
 
diff --git a/compiler/testData/diagnostics/tests/IncDec.fir.kt b/compiler/testData/diagnostics/tests/IncDec.fir.kt
index 00153790c68..fa8326da3a6 100644
--- a/compiler/testData/diagnostics/tests/IncDec.fir.kt
+++ b/compiler/testData/diagnostics/tests/IncDec.fir.kt
@@ -16,8 +16,8 @@ fun testIncDec() {
 }
 
 class WrongIncDec() {
-  operator fun inc() : Int = 1
-  operator fun dec() : Int = 1
+  <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun inc() : Int = 1
+  <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun dec() : Int = 1
 }
 
 fun testWrongIncDec() {
@@ -29,8 +29,8 @@ fun testWrongIncDec() {
 }
 
 class UnitIncDec() {
-  operator fun inc() : Unit {}
-  operator fun dec() : Unit {}
+  <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun inc() : Unit {}
+  <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun dec() : Unit {}
 }
 
 fun testUnitIncDec() {
diff --git a/compiler/testData/diagnostics/tests/OperatorChecks.fir.kt b/compiler/testData/diagnostics/tests/OperatorChecks.fir.kt
index 740a20d4346..c344e2be272 100644
--- a/compiler/testData/diagnostics/tests/OperatorChecks.fir.kt
+++ b/compiler/testData/diagnostics/tests/OperatorChecks.fir.kt
@@ -5,59 +5,59 @@ import kotlin.reflect.KProperty
 interface Example {
     operator fun plus(o: Example): Example
     operator fun div(o: Example): Example
-    operator fun plus(o: Example, s: String = ""): Example
-    operator fun minus(vararg o: Example): Example
-    operator fun plus(): Example
-    operator fun minus(): Example
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun plus(o: Example, s: String = ""): Example
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun minus(vararg o: Example): Example
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun plus(): Example
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun minus(): Example
 
     operator fun unaryPlus(): Example
     operator fun unaryMinus(): Example
 
-    operator fun unaryPlus(s: String = ""): Example
-    operator fun unaryMinus(o: Example)
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun unaryPlus(s: String = ""): Example
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun unaryMinus(o: Example)
 
     operator fun inc(): Example
-    operator fun dec(): Example?
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun dec(): Example?
 
     operator fun plusAssign(n: Int)
-    operator fun minusAssign(n: Int): String
-    operator fun divAssign(n: Int, a: String = "")
-    operator fun modAssign(vararg n: Int)
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun minusAssign(n: Int): String
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun divAssign(n: Int, a: String = "")
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun modAssign(vararg n: Int)
 
     operator fun compareTo(other: Example): Int
 
     override operator fun equals(other: Any?): Boolean
-    operator fun equals(a: String): Boolean
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun equals(a: String): Boolean
 
     operator fun contains(n: Int): Boolean
-    operator fun contains(n: Int, s: String = ""): Boolean
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun contains(n: Int, s: String = ""): Boolean
 
     operator fun invoke()
 
     operator fun get(n: Int)
     operator fun get(n: Int, n2: Int)
-    operator fun get()
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun get()
 
     operator fun set(n: Int, v: Int)
     operator fun set(n: Int, n2: Int, v: Int)
-    operator fun set(v: Int)
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun set(v: Int)
 
     operator fun rangeTo(o: Int)
-    operator fun rangeTo(o: Int, o2: Int)
-    operator fun rangeTo(vararg o: String)
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun rangeTo(o: Int, o2: Int)
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun rangeTo(vararg o: String)
 
     operator fun component1(): Int
-    operator fun component1(n: Int): Int
-    operator fun componentN(): Int
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun component1(n: Int): Int
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun componentN(): Int
 
     operator fun iterator(): String
-    operator fun iterator(n: Int): String
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun iterator(n: Int): String
 
     operator fun next(): String
-    operator fun next(n: Int): String
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun next(n: Int): String
 
     operator fun hasNext(): Boolean
-    operator fun hasNext(n: Int): String
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun hasNext(n: Int): String
 
     infix fun i1(n: Int)
     <!INAPPLICABLE_INFIX_MODIFIER!>infix fun i1(n: Int, n2: Int)<!>
@@ -72,27 +72,27 @@ class OkDelegates {
 }
 
 class DelegatesWithErrors {
-    operator fun getValue(thisRef: Any?, prop: String): String = ""
-    operator fun setValue(thisRef: Any?, prop: String, value: String) {}
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun getValue(thisRef: Any?, prop: String): String = ""
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun setValue(thisRef: Any?, prop: String, value: String) {}
 
-    operator fun setValue(thisRef: Any?, prop: KProperty<*>, vararg n: Int) {}
-    operator fun setValue(thisRef: Any?, prop: KProperty<*>, f: Float = 0.0f) {}
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun setValue(thisRef: Any?, prop: KProperty<*>, vararg n: Int) {}
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun setValue(thisRef: Any?, prop: KProperty<*>, f: Float = 0.0f) {}
 
-    operator fun getValue(prop: KProperty<*>): String = ""
-    operator fun setValue(prop: KProperty<*>, value: String) {}
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun getValue(prop: KProperty<*>): String = ""
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun setValue(prop: KProperty<*>, value: String) {}
 }
 
 interface Example2 {
-    operator fun inc(s: String): Example
-    operator fun dec()
-    operator fun compareTo(vararg other: Example): Int
-    operator fun contains(vararg n: Int): Boolean
-    operator fun hasNext(): Int
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun inc(s: String): Example
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun dec()
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun compareTo(vararg other: Example): Int
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun contains(vararg n: Int): Boolean
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun hasNext(): Int
 }
 
 interface Example3 {
-    operator fun compareTo(other: Example, s: String = ""): Int
-    operator fun contains(n: Int)
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun compareTo(other: Example, s: String = ""): Int
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun contains(n: Int)
 }
 
 
@@ -100,58 +100,58 @@ interface Example3 {
 
 operator fun Example.plus(o: Any): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
 operator fun Example.div(o: Example): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
-operator fun Example.plus(o: Example, s: String = ""): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
-operator fun Example.minus(vararg o: Example): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
-operator fun Example.plus(): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
-operator fun Example.minus(): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun Example.plus(o: Example, s: String = ""): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun Example.minus(vararg o: Example): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun Example.plus(): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun Example.minus(): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
 
 operator fun Example.unaryPlus(): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
 operator fun Example.unaryMinus(): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
 
-operator fun Example.unaryPlus(s: String = ""): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
-operator fun Example.unaryMinus(o: Example) {}
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun Example.unaryPlus(s: String = ""): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun Example.unaryMinus(o: Example) {}
 
 operator fun Example.inc(): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
-operator fun Example.dec(): Example? {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun Example.dec(): Example? {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
 
 operator fun Example.plusAssign(n: Int) {}
-operator fun Example.minusAssign(n: Int): String {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
-operator fun Example.divAssign(n: Int, a: String = "") {}
-operator fun Example.modAssign(vararg n: Int) {}
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun Example.minusAssign(n: Int): String {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun Example.divAssign(n: Int, a: String = "") {}
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun Example.modAssign(vararg n: Int) {}
 
 operator fun Example.compareTo(other: Example): Int {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
 
-operator fun Example.equals(a: String): Boolean {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun Example.equals(a: String): Boolean {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
 
 operator fun Example.contains(n: Int): Boolean {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
-operator fun Example.contains(n: Int, s: String = ""): Boolean {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun Example.contains(n: Int, s: String = ""): Boolean {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
 
 operator fun Example.invoke() {}
 
 operator fun Example.get(n: Int) {}
 operator fun Example.get(n: Int, n2: Int) {}
-operator fun Example.get() {}
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun Example.get() {}
 
 operator fun Example.set(n: Int, v: Int) {}
 operator fun Example.set(n: Int, n2: Int, v: Int) {}
-operator fun Example.set(v: Int) {}
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun Example.set(v: Int) {}
 
 operator fun Example.rangeTo(o: Int) {}
-operator fun Example.rangeTo(o: Int, o2: Int) {}
-operator fun Example.rangeTo(vararg o: String) {}
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun Example.rangeTo(o: Int, o2: Int) {}
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun Example.rangeTo(vararg o: String) {}
 
 operator fun Example.component1(): Int {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
-operator fun Example.component1(n: Int): Int {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
-operator fun Example.componentN(): Int {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun Example.component1(n: Int): Int {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun Example.componentN(): Int {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
 
 operator fun Example.iterator(): String {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
-operator fun Example.iterator(n: Int): String {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun Example.iterator(n: Int): String {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
 
 operator fun Example.next(): String {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
-operator fun Example.next(n: Int): String {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun Example.next(n: Int): String {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
 
 operator fun Example.hasNext(): Boolean {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
-operator fun Example.hasNext(n: Int): String {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun Example.hasNext(n: Int): String {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
 
 infix fun Example.i1(n: Int) {}
 <!INAPPLICABLE_INFIX_MODIFIER!>infix fun Example.i1(n: Int, n2: Int) {}<!>
@@ -161,58 +161,58 @@ infix fun Example.i1(vararg n: Int) {}
 
 
 
-operator fun plus(o: String): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
-operator fun div(o: Example): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
-operator fun plus(o: Example, s: String = ""): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
-operator fun minus(vararg o: Example): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun plus(o: String): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun div(o: Example): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun plus(o: Example, s: String = ""): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun minus(vararg o: Example): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
 
-operator fun unaryPlus(): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
-operator fun unaryMinus(): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun unaryPlus(): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun unaryMinus(): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
 
-operator fun unaryPlus(s: String = ""): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
-operator fun unaryMinus(o: Example) {}
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun unaryPlus(s: String = ""): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun unaryMinus(o: Example) {}
 
-operator fun inc(): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
-operator fun dec(): Example? {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun inc(): Example {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun dec(): Example? {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
 
-operator fun plusAssign(n: Int) {}
-operator fun minusAssign(n: Int): String {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
-operator fun divAssign(n: Int, a: String = "") {}
-operator fun modAssign(vararg n: Int) {}
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun plusAssign(n: Int) {}
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun minusAssign(n: Int): String {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun divAssign(n: Int, a: String = "") {}
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun modAssign(vararg n: Int) {}
 
-operator fun compareTo(other: Example): Int {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun compareTo(other: Example): Int {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
 
-operator fun equals(a: String): Boolean {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun equals(a: String): Boolean {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
 
-operator fun contains(n: Int): Boolean {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
-operator fun contains(n: Int, s: String = ""): Boolean {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun contains(n: Int): Boolean {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun contains(n: Int, s: String = ""): Boolean {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
 
-operator fun invoke() {}
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun invoke() {}
 
-operator fun get(n: Int) {}
-operator fun get(n: Int, n2: Int) {}
-operator fun get() {}
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun get(n: Int) {}
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun get(n: Int, n2: Int) {}
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun get() {}
 
-operator fun set(n: Int, v: Int) {}
-operator fun set(n: Int, n2: Int, v: Int) {}
-operator fun set(v: Int) {}
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun set(n: Int, v: Int) {}
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun set(n: Int, n2: Int, v: Int) {}
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun set(v: Int) {}
 
-operator fun rangeTo(o: Int) {}
-operator fun rangeTo(o: Int, o2: Int) {}
-operator fun rangeTo(vararg o: String) {}
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun rangeTo(o: Int) {}
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun rangeTo(o: Int, o2: Int) {}
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun rangeTo(vararg o: String) {}
 
-operator fun component1(): Int {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
-operator fun component1(n: Int): Int {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
-operator fun componentN(): Int {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun component1(): Int {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun component1(n: Int): Int {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun componentN(): Int {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
 
-operator fun iterator(): String {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
-operator fun iterator(n: Int): String {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun iterator(): String {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun iterator(n: Int): String {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
 
-operator fun next(): String {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
-operator fun next(n: Int): String {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun next(): String {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun next(n: Int): String {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
 
-operator fun hasNext(): Boolean {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
-operator fun hasNext(n: Int): String {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun hasNext(): Boolean {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun hasNext(n: Int): String {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
 
 <!INAPPLICABLE_INFIX_MODIFIER!>infix fun i1(n: Int) {}<!>
 <!INAPPLICABLE_INFIX_MODIFIER!>infix fun i1(n: Int, n2: Int) {}<!>
diff --git a/compiler/testData/diagnostics/tests/OperatorsWithWrongNames.fir.kt b/compiler/testData/diagnostics/tests/OperatorsWithWrongNames.fir.kt
deleted file mode 100644
index 16440292185..00000000000
--- a/compiler/testData/diagnostics/tests/OperatorsWithWrongNames.fir.kt
+++ /dev/null
@@ -1,11 +0,0 @@
-class A {
-    operator fun A.minus(o: A) = o
-
-    operator fun A.add(o: A) = o
-    operator fun A.get(o: A) = o
-    operator fun A.invokee() {}
-}
-
-operator fun A.plus(o: A) = o
-operator fun A.component1() = 1
-operator fun A.componentN() = 1
\ No newline at end of file
diff --git a/compiler/testData/diagnostics/tests/OperatorsWithWrongNames.kt b/compiler/testData/diagnostics/tests/OperatorsWithWrongNames.kt
index 142d53068e8..47dc3cce0f0 100644
--- a/compiler/testData/diagnostics/tests/OperatorsWithWrongNames.kt
+++ b/compiler/testData/diagnostics/tests/OperatorsWithWrongNames.kt
@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 class A {
     operator fun A.minus(o: A) = o
 
diff --git a/compiler/testData/diagnostics/tests/checkArguments/arrayAccessSet.fir.kt b/compiler/testData/diagnostics/tests/checkArguments/arrayAccessSet.fir.kt
index 68cff469d7c..085af99f3d6 100644
--- a/compiler/testData/diagnostics/tests/checkArguments/arrayAccessSet.fir.kt
+++ b/compiler/testData/diagnostics/tests/checkArguments/arrayAccessSet.fir.kt
@@ -16,7 +16,7 @@ object D {
 }
 
 object Z {
-    operator fun set() {
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun set() {
     }
 }
 
diff --git a/compiler/testData/diagnostics/tests/delegatedProperty/inference/noErrorsForImplicitConstraints.fir.kt b/compiler/testData/diagnostics/tests/delegatedProperty/inference/noErrorsForImplicitConstraints.fir.kt
index a9081d1a357..914e317e5d0 100644
--- a/compiler/testData/diagnostics/tests/delegatedProperty/inference/noErrorsForImplicitConstraints.fir.kt
+++ b/compiler/testData/diagnostics/tests/delegatedProperty/inference/noErrorsForImplicitConstraints.fir.kt
@@ -35,7 +35,7 @@ class MyProperty2<T, R> {
         throw Exception()
     }
 
-    operator fun setValue(i: Int) {
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun setValue(i: Int) {
         println("set")
     }
 }
diff --git a/compiler/testData/diagnostics/tests/delegatedProperty/provideDelegate/provideDelegateOperatorDeclaration.fir.kt b/compiler/testData/diagnostics/tests/delegatedProperty/provideDelegate/provideDelegateOperatorDeclaration.fir.kt
deleted file mode 100644
index 1bcc06edadb..00000000000
--- a/compiler/testData/diagnostics/tests/delegatedProperty/provideDelegate/provideDelegateOperatorDeclaration.fir.kt
+++ /dev/null
@@ -1,32 +0,0 @@
-// !DIAGNOSTICS: -UNUSED_PARAMETER
-
-import kotlin.reflect.KProperty
-
-operator fun provideDelegate(x: Any?, p: KProperty<*>) {}
-
-operator fun Any.provideDelegate(x: Any?, p: KProperty<*>) {}
-
-operator fun Any.provideDelegate(x: Any?, p: Any) {}
-
-operator fun Any.provideDelegate(x: Any?, p: Int) {}
-
-class Host1 {
-    operator fun provideDelegate(x: Any?, p: KProperty<*>) {}
-}
-
-class Host2 {
-    operator fun Any.provideDelegate(x: Any?, p: KProperty<*>) {}
-}
-
-class Host3 {
-    operator fun provideDelegate(x: Any?, p: KProperty<*>, foo: Int) {}
-}
-
-class Host4 {
-    operator fun provideDelegate(x: Any?, p: KProperty<*>, foo: Int = 0) {}
-}
-
-class Host5 {
-    operator fun provideDelegate(x: Any?, p: KProperty<*>, vararg foo: Int) {}
-}
-
diff --git a/compiler/testData/diagnostics/tests/delegatedProperty/provideDelegate/provideDelegateOperatorDeclaration.kt b/compiler/testData/diagnostics/tests/delegatedProperty/provideDelegate/provideDelegateOperatorDeclaration.kt
index e620326a94b..ef980988ef3 100644
--- a/compiler/testData/diagnostics/tests/delegatedProperty/provideDelegate/provideDelegateOperatorDeclaration.kt
+++ b/compiler/testData/diagnostics/tests/delegatedProperty/provideDelegate/provideDelegateOperatorDeclaration.kt
@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // !DIAGNOSTICS: -UNUSED_PARAMETER
 
 import kotlin.reflect.KProperty
diff --git a/compiler/testData/diagnostics/tests/inline/binaryExpressions/assignment.fir.kt b/compiler/testData/diagnostics/tests/inline/binaryExpressions/assignment.fir.kt
index 0324a66e8aa..7182ae34917 100644
--- a/compiler/testData/diagnostics/tests/inline/binaryExpressions/assignment.fir.kt
+++ b/compiler/testData/diagnostics/tests/inline/binaryExpressions/assignment.fir.kt
@@ -2,7 +2,7 @@
 
 operator fun <T, U> Function1<T, U>.minusAssign(p: Function1<T, U>) {}
 
-inline operator fun <T, U> Function1<T, U>.modAssign(p: Function1<T, U>) = {
+inline <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun <T, U> Function1<T, U>.modAssign(p: Function1<T, U>) = {
     this += p
     p += this
 }
@@ -14,7 +14,7 @@ inline operator fun <T, U> Function1<T, U>.plusAssign(p: Function1<T, U>) {
 
 operator fun <T, U, V> @ExtensionFunctionType Function2<T, U, V>.minusAssign(ext : @ExtensionFunctionType Function2<T, U, V>) {}
 
-inline operator fun <T, U, V> @ExtensionFunctionType Function2<T, U, V>.modAssign(ext : @ExtensionFunctionType Function2<T, U, V>) = {
+inline <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun <T, U, V> @ExtensionFunctionType Function2<T, U, V>.modAssign(ext : @ExtensionFunctionType Function2<T, U, V>) = {
     this += ext
     ext += this
 }
diff --git a/compiler/testData/diagnostics/tests/operatorsOverloading/compareToNullable.fir.kt b/compiler/testData/diagnostics/tests/operatorsOverloading/compareToNullable.fir.kt
index a400f76638a..6ecf1d0f698 100644
--- a/compiler/testData/diagnostics/tests/operatorsOverloading/compareToNullable.fir.kt
+++ b/compiler/testData/diagnostics/tests/operatorsOverloading/compareToNullable.fir.kt
@@ -1,7 +1,7 @@
 // !DIAGNOSTICS: -UNUSED_PARAMETER
 
 class C {
-    operator fun compareTo(c: C): Int? = null
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun compareTo(c: C): Int? = null
 }
 
 fun test(c: C) {
@@ -9,4 +9,4 @@ fun test(c: C) {
     c <= c
     c >= c
     c > c
-}
\ No newline at end of file
+}
diff --git a/compiler/testData/diagnostics/tests/operatorsOverloading/kt1028.fir.kt b/compiler/testData/diagnostics/tests/operatorsOverloading/kt1028.fir.kt
deleted file mode 100644
index e4f9c170868..00000000000
--- a/compiler/testData/diagnostics/tests/operatorsOverloading/kt1028.fir.kt
+++ /dev/null
@@ -1,35 +0,0 @@
-//KT-1028 Wrong type checking for plusAssign
-package kt1028
-
-import java.util.*
-
-class event<T>()
-{
-    val callbacks = ArrayList< Function1<T, Unit> >() // Should be ArrayList<()->Unit>, bug posted
-
-    operator fun plusAssign(f : (T) -> Unit) = callbacks.add(f)
-    operator fun minusAssign(f : (T) -> Unit) = callbacks.remove(f)
-    fun call(value : T) { for(c in callbacks) c(value) }
-}
-
-class MouseMovedEventArgs()
-{
-    public val X : Int = 0
-}
-
-class Control()
-{
-    public val MouseMoved : event<MouseMovedEventArgs> = event<MouseMovedEventArgs>()
-
-    fun MoveMouse() = MouseMoved.call(MouseMovedEventArgs())
-}
-
-class Test()
-{
-    fun test()
-    {
-        val control = Control()
-        control.MouseMoved <!ASSIGNMENT_OPERATOR_SHOULD_RETURN_UNIT!>+=<!> { it.X } // here
-        control.MouseMoved.plusAssign( { it.X } ) // ok
-    }
-}
diff --git a/compiler/testData/diagnostics/tests/operatorsOverloading/kt1028.kt b/compiler/testData/diagnostics/tests/operatorsOverloading/kt1028.kt
index 18169a7e199..28727f1d52c 100644
--- a/compiler/testData/diagnostics/tests/operatorsOverloading/kt1028.kt
+++ b/compiler/testData/diagnostics/tests/operatorsOverloading/kt1028.kt
@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 //KT-1028 Wrong type checking for plusAssign
 package kt1028
 
diff --git a/compiler/testData/diagnostics/tests/regressions/kt618.fir.kt b/compiler/testData/diagnostics/tests/regressions/kt618.fir.kt
deleted file mode 100644
index b631c9dee80..00000000000
--- a/compiler/testData/diagnostics/tests/regressions/kt618.fir.kt
+++ /dev/null
@@ -1,28 +0,0 @@
-package lol
-
-class B() {
-    operator fun plusAssign(other : B) : String {
-        return "s"
-    }
-    operator fun minusAssign(other : B) : String {
-        return "s"
-    }
-    operator fun remAssign(other : B) : String {
-        return "s"
-    }
-    operator fun divAssign(other : B) : String {
-        return "s"
-    }
-    operator fun timesAssign(other : B) : String {
-        return "s"
-    }
-}
-
-fun main() {
-    var c = B()
-    c <!ASSIGNMENT_OPERATOR_SHOULD_RETURN_UNIT!>+=<!> B()
-    c <!ASSIGNMENT_OPERATOR_SHOULD_RETURN_UNIT!>*=<!> B()
-    c <!ASSIGNMENT_OPERATOR_SHOULD_RETURN_UNIT!>/=<!> B()
-    c <!ASSIGNMENT_OPERATOR_SHOULD_RETURN_UNIT!>-=<!> B()
-    c <!ASSIGNMENT_OPERATOR_SHOULD_RETURN_UNIT!>%=<!> B()
-}
diff --git a/compiler/testData/diagnostics/tests/regressions/kt618.kt b/compiler/testData/diagnostics/tests/regressions/kt618.kt
index 02a2bd8cab8..9fb8d87371d 100644
--- a/compiler/testData/diagnostics/tests/regressions/kt618.kt
+++ b/compiler/testData/diagnostics/tests/regressions/kt618.kt
@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 package lol
 
 class B() {
diff --git a/compiler/testData/diagnostics/tests/smartCasts/incDecToNull.fir.kt b/compiler/testData/diagnostics/tests/smartCasts/incDecToNull.fir.kt
deleted file mode 100644
index d693218c103..00000000000
--- a/compiler/testData/diagnostics/tests/smartCasts/incDecToNull.fir.kt
+++ /dev/null
@@ -1,10 +0,0 @@
-class IncDec {
-    operator fun inc(): Unit {}
-}
-
-fun foo(): IncDec {
-    var x = IncDec()
-    x = x<!INC_DEC_SHOULD_NOT_RETURN_UNIT!>++<!>
-    x<!INC_DEC_SHOULD_NOT_RETURN_UNIT!>++<!>
-    return x
-}
diff --git a/compiler/testData/diagnostics/tests/smartCasts/incDecToNull.kt b/compiler/testData/diagnostics/tests/smartCasts/incDecToNull.kt
index 598fd1580b3..67924d7ffc9 100644
--- a/compiler/testData/diagnostics/tests/smartCasts/incDecToNull.kt
+++ b/compiler/testData/diagnostics/tests/smartCasts/incDecToNull.kt
@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 class IncDec {
     <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun inc(): Unit {}
 }
diff --git a/compiler/testData/diagnostics/testsWithStdLib/coroutines/operators.fir.kt b/compiler/testData/diagnostics/testsWithStdLib/coroutines/operators.fir.kt
index 9a9b4df845a..2f83974c558 100644
--- a/compiler/testData/diagnostics/testsWithStdLib/coroutines/operators.fir.kt
+++ b/compiler/testData/diagnostics/testsWithStdLib/coroutines/operators.fir.kt
@@ -32,7 +32,7 @@ class A {
 
     suspend operator fun contains(b: A) = this == b
     suspend operator fun get(a: A) = a
-    suspend operator fun equals(a: A) = a === this
+    suspend <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun equals(a: A) = a === this
     suspend operator fun set(a: A, b: A) {}
 
     suspend operator fun provideDelegate(a: A, p: KProperty<*>) = a
diff --git a/compiler/tests-spec/testData/diagnostics/linked/expressions/comparison-expressions/p-4/neg/1.1.fir.kt b/compiler/tests-spec/testData/diagnostics/linked/expressions/comparison-expressions/p-4/neg/1.1.fir.kt
index 884ce5531ea..5bb5af05cdd 100644
--- a/compiler/tests-spec/testData/diagnostics/linked/expressions/comparison-expressions/p-4/neg/1.1.fir.kt
+++ b/compiler/tests-spec/testData/diagnostics/linked/expressions/comparison-expressions/p-4/neg/1.1.fir.kt
@@ -5,7 +5,7 @@
 // TESTCASE NUMBER: 1
 class Case1(val a: Int)  {
     var isCompared = false
-    operator fun compareTo(other: Case1):Any = run{
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun compareTo(other: Case1):Any = run{
         TODO()
     }
 }
@@ -24,7 +24,7 @@ fun case1() {
 // TESTCASE NUMBER: 2
 class Case2(val a: Int) {
     var isCompared = false
-    operator fun compareTo(other: Case2): Nothing = run {
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun compareTo(other: Case2): Nothing = run {
         TODO()
     }
 }
@@ -43,7 +43,7 @@ fun case2() {
 // TESTCASE NUMBER: 3
 class Case3(val a: Int) {
     var isCompared = false
-    operator fun compareTo(other: Case3):Long = run{
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun compareTo(other: Case3):Long = run{
         TODO()
     }
 }
@@ -61,7 +61,7 @@ fun case3() {
 // TESTCASE NUMBER: 4
 class Case4(val a: Int) {
     var isCompared = false
-    operator fun compareTo(other: Case4):Int? = run{
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun compareTo(other: Case4):Int? = run{
         TODO()
     }
 }
diff --git a/compiler/tests-spec/testData/diagnostics/linked/expressions/prefix-expressions/prefix-decrement-expression/p-5/neg/2.1.fir.kt b/compiler/tests-spec/testData/diagnostics/linked/expressions/prefix-expressions/prefix-decrement-expression/p-5/neg/2.1.fir.kt
index 512e8850fba..ef122218f1e 100644
--- a/compiler/tests-spec/testData/diagnostics/linked/expressions/prefix-expressions/prefix-decrement-expression/p-5/neg/2.1.fir.kt
+++ b/compiler/tests-spec/testData/diagnostics/linked/expressions/prefix-expressions/prefix-decrement-expression/p-5/neg/2.1.fir.kt
@@ -12,7 +12,7 @@ fun case1() {
 
 class Case1() {
 
-    operator fun inc(): B {
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun inc(): B {
         TODO()
     }
 }
@@ -29,7 +29,7 @@ fun case2() {
 class Case2() : C() {
     var i = 0
 
-    operator fun inc(): C {
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun inc(): C {
         TODO()
     }
 
diff --git a/compiler/tests-spec/testData/diagnostics/linked/expressions/prefix-expressions/prefix-increment-expression/p-5/neg/2.1.fir.kt b/compiler/tests-spec/testData/diagnostics/linked/expressions/prefix-expressions/prefix-increment-expression/p-5/neg/2.1.fir.kt
index e0624495864..00791794406 100644
--- a/compiler/tests-spec/testData/diagnostics/linked/expressions/prefix-expressions/prefix-increment-expression/p-5/neg/2.1.fir.kt
+++ b/compiler/tests-spec/testData/diagnostics/linked/expressions/prefix-expressions/prefix-increment-expression/p-5/neg/2.1.fir.kt
@@ -12,7 +12,7 @@ fun case1() {
 
 class Case1() {
 
-    operator fun dec(): B {
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun dec(): B {
         TODO()
     }
 }
@@ -29,7 +29,7 @@ fun case2() {
 class Case2() : C() {
     var i = 0
 
-    operator fun dec(): C {
+    <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun dec(): C {
         TODO()
     }
 
diff --git a/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDataClassConverters.kt b/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDataClassConverters.kt
index c2edaa4c62e..96c7ac16452 100644
--- a/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDataClassConverters.kt
+++ b/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDataClassConverters.kt
@@ -1099,6 +1099,13 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.INAPPLICABLE_OPERATOR_MODIFIER) { firDiagnostic ->
+        InapplicableOperatorModifierImpl(
+            firDiagnostic.a,
+            firDiagnostic as FirPsiDiagnostic,
+            token,
+        )
+    }
     add(FirErrors.INLINE_CLASS_NOT_TOP_LEVEL) { firDiagnostic ->
         InlineClassNotTopLevelImpl(
             firDiagnostic as FirPsiDiagnostic,
diff --git a/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnostics.kt b/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnostics.kt
index f7e76520b25..1c7acec00a9 100644
--- a/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnostics.kt
+++ b/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnostics.kt
@@ -796,6 +796,11 @@ sealed class KtFirDiagnostic<PSI : PsiElement> : KtDiagnosticWithPsi<PSI> {
         abstract val target: String
     }
 
+    abstract class InapplicableOperatorModifier : KtFirDiagnostic<PsiElement>() {
+        override val diagnosticClass get() = InapplicableOperatorModifier::class
+        abstract val message: String
+    }
+
     abstract class InlineClassNotTopLevel : KtFirDiagnostic<KtDeclaration>() {
         override val diagnosticClass get() = InlineClassNotTopLevel::class
     }
diff --git a/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnosticsImpl.kt b/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnosticsImpl.kt
index 7baa7992ef6..a01ae61c23f 100644
--- a/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnosticsImpl.kt
+++ b/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnosticsImpl.kt
@@ -1262,6 +1262,14 @@ internal class DeprecatedModifierContainingDeclarationImpl(
     override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
 }
 
+internal class InapplicableOperatorModifierImpl(
+    override val message: String,
+    firDiagnostic: FirPsiDiagnostic,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.InapplicableOperatorModifier(), KtAbstractFirDiagnostic<PsiElement> {
+    override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
+}
+
 internal class InlineClassNotTopLevelImpl(
     firDiagnostic: FirPsiDiagnostic,
     override val token: ValidityToken,