FIR: report ENUM_ENTRY_AS_TYPE and IS_ENUM_TYPE

These two diagnostics are similar: both are reported on type references
to enum entries. But `IS_ENUM_TYPE` is reported if the type ref is an
operand of `is` operator. To pass along this contextual information, a
boolean is added to FirSpecificTypeResolverTransformer.

compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/FirDiagnosticsList.kt

@@ -966,6 +966,8 @@ object DIAGNOSTICS_LIST : DiagnosticList("FirErrors") {
         val USELESS_IS_CHECK by warning<KtElement> {
             parameter<Boolean>("compileTimeCheckResult")
         }
+        val IS_ENUM_ENTRY by error<KtTypeReference>()
+        val ENUM_ENTRY_AS_TYPE by error<KtTypeReference>()
     }
 
     val WHEN_EXPRESSIONS by object : DiagnosticGroup("When expressions") {

compiler/fir/checkers/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/FirErrors.kt

@@ -509,6 +509,8 @@ object FirErrors {
     // Casts and is-checks
     val USELESS_CAST by warning0<KtBinaryExpressionWithTypeRHS>(SourceElementPositioningStrategies.AS_TYPE)
     val USELESS_IS_CHECK by warning1<KtElement, Boolean>()
+    val IS_ENUM_ENTRY by error0<KtTypeReference>()
+    val ENUM_ENTRY_AS_TYPE by error0<KtTypeReference>()
 
     // When expressions
     val EXPECTED_CONDITION by error0<KtWhenCondition>()

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirDefaultErrorMessages.kt

@@ -131,6 +131,7 @@ import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.DEPRECATION_ERROR
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.DESERIALIZATION_ERROR
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.DYNAMIC_UPPER_BOUND
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.EMPTY_RANGE
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.ENUM_ENTRY_AS_TYPE
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.EQUALITY_NOT_APPLICABLE
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.EQUALITY_NOT_APPLICABLE_WARNING
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.ERROR_FROM_JAVA_RESOLUTION
@@ -223,6 +224,7 @@ import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INTERFACE_WITH_SU
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INVALID_IF_AS_EXPRESSION
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INVALID_TYPE_OF_ANNOTATION_MEMBER
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INVISIBLE_REFERENCE
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.IS_ENUM_ENTRY
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.ITERATOR_AMBIGUITY
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.KCLASS_WITH_NULLABLE_TYPE_PARAMETER_IN_SIGNATURE
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.LEAKED_IN_PLACE_LAMBDA
@@ -1276,6 +1278,8 @@ class FirDefaultErrorMessages {
             // Casts and is-checks
             map.put(USELESS_CAST, "No cast needed")
             map.put(USELESS_IS_CHECK, "Check for instance is always ''{0}''", TO_STRING)
+            map.put(IS_ENUM_ENTRY, "'is' over enum entry is not allowed, use comparison instead")
+            map.put(ENUM_ENTRY_AS_TYPE, "Use of enum entry names as types is not allowed, use enum type instead")
 
             // When expressions
             map.put(EXPECTED_CONDITION, "Expected condition of type Boolean")

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/coneDiagnosticToFirDiagnostic.kt

@@ -359,6 +359,8 @@ private fun ConeSimpleDiagnostic.getFactory(source: FirSourceElement): FirDiagno
         DiagnosticKind.RecursiveTypealiasExpansion -> FirErrors.RECURSIVE_TYPEALIAS_EXPANSION
         DiagnosticKind.LoopInSupertype -> FirErrors.CYCLIC_INHERITANCE_HIERARCHY
         DiagnosticKind.IllegalSelector -> FirErrors.ILLEGAL_SELECTOR
+        DiagnosticKind.IsEnumEntry -> FirErrors.IS_ENUM_ENTRY
+        DiagnosticKind.EnumEntryAsType -> FirErrors.ENUM_ENTRY_AS_TYPE
         DiagnosticKind.UnresolvedSupertype,
         DiagnosticKind.UnresolvedExpandedType,
         DiagnosticKind.Other -> FirErrors.OTHER_ERROR

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/FirTypeResolver.kt

@@ -11,5 +11,5 @@ import org.jetbrains.kotlin.fir.types.ConeKotlinType
 import org.jetbrains.kotlin.fir.types.FirTypeRef
 
 abstract class FirTypeResolver : FirSessionComponent {
-    abstract fun resolveType(typeRef: FirTypeRef, scopeClassDeclaration: ScopeClassDeclaration, areBareTypesAllowed: Boolean): ConeKotlinType
+    abstract fun resolveType(typeRef: FirTypeRef, scopeClassDeclaration: ScopeClassDeclaration, areBareTypesAllowed: Boolean, isOperandOfIsOperator: Boolean): ConeKotlinType
 }

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/providers/impl/FirTypeResolverImpl.kt

@@ -6,11 +6,18 @@
 package org.jetbrains.kotlin.fir.resolve.providers.impl
 
 import org.jetbrains.kotlin.builtins.StandardNames
-import org.jetbrains.kotlin.fir.*
+import org.jetbrains.kotlin.fir.FirSession
-import org.jetbrains.kotlin.fir.declarations.*
+import org.jetbrains.kotlin.fir.FirSourceElement
+import org.jetbrains.kotlin.fir.ThreadSafeMutableState
+import org.jetbrains.kotlin.fir.declarations.FirEnumEntry
+import org.jetbrains.kotlin.fir.declarations.FirRegularClass
 import org.jetbrains.kotlin.fir.declarations.impl.FirOuterClassTypeParameterRef
 import org.jetbrains.kotlin.fir.declarations.utils.classId
+import org.jetbrains.kotlin.fir.declarations.utils.isEnumClass
+import org.jetbrains.kotlin.fir.diagnostics.ConeSimpleDiagnostic
 import org.jetbrains.kotlin.fir.diagnostics.ConeUnexpectedTypeArgumentsError
+import org.jetbrains.kotlin.fir.diagnostics.DiagnosticKind
+import org.jetbrains.kotlin.fir.render
 import org.jetbrains.kotlin.fir.resolve.*
 import org.jetbrains.kotlin.fir.resolve.diagnostics.ConeOuterClassArgumentsRequired
 import org.jetbrains.kotlin.fir.resolve.diagnostics.ConeUnresolvedQualifierError
@@ -20,6 +27,8 @@ import org.jetbrains.kotlin.fir.resolve.substitution.ConeSubstitutor
 import org.jetbrains.kotlin.fir.resolve.transformers.ScopeClassDeclaration
 import org.jetbrains.kotlin.fir.scopes.FirScope
 import org.jetbrains.kotlin.fir.symbols.ConeTypeParameterLookupTag
+import org.jetbrains.kotlin.fir.symbols.FirBasedSymbol
+import org.jetbrains.kotlin.fir.symbols.SymbolInternals
 import org.jetbrains.kotlin.fir.symbols.impl.*
 import org.jetbrains.kotlin.fir.types.*
 import org.jetbrains.kotlin.fir.types.impl.ConeClassLikeTypeImpl
@@ -49,7 +58,7 @@ class FirTypeResolverImpl(private val session: FirSession) : FirTypeResolver() {
     private fun resolveToSymbol(
         typeRef: FirTypeRef,
         scope: FirScope,
-    ): Pair<FirClassifierSymbol<*>?, ConeSubstitutor?> {
+    ): Pair<FirBasedSymbol<*>?, ConeSubstitutor?> {
         return when (typeRef) {
             is FirResolvedTypeRef -> {
                 val resultSymbol = typeRef.coneTypeSafe<ConeLookupTagBasedType>()?.lookupTag?.let(symbolProvider::getSymbolByLookupTag)
@@ -58,7 +67,7 @@ class FirTypeResolverImpl(private val session: FirSession) : FirTypeResolver() {
 
             is FirUserTypeRef -> {
                 val qualifierResolver = session.qualifierResolver
-                var resolvedSymbol: FirClassifierSymbol<*>? = null
+                var resolvedSymbol: FirBasedSymbol<*>? = null
                 var substitutor: ConeSubstitutor? = null
                 scope.processClassifiersByNameWithSubstitution(typeRef.qualifier.first().name) { symbol, substitutorFromScope ->
                     if (resolvedSymbol != null) return@processClassifiersByNameWithSubstitution
@@ -68,6 +77,7 @@ class FirTypeResolverImpl(private val session: FirSession) : FirTypeResolver() {
                                 symbol
                             } else {
                                 qualifierResolver.resolveSymbolWithPrefix(typeRef.qualifier, symbol.classId)
+                                    ?: qualifierResolver.resolveEnumEntrySymbol(typeRef.qualifier, symbol.classId)
                             }
                         }
                         is FirTypeParameterSymbol -> {
@@ -80,7 +90,7 @@ class FirTypeResolverImpl(private val session: FirSession) : FirTypeResolver() {
                 }
 
                 // TODO: Imports
-                val resultSymbol: FirClassifierSymbol<*>? = resolvedSymbol ?: qualifierResolver.resolveSymbol(typeRef.qualifier)
+                val resultSymbol: FirBasedSymbol<*>? = resolvedSymbol ?: qualifierResolver.resolveSymbol(typeRef.qualifier)
                 resultSymbol to substitutor
             }
 
@@ -92,15 +102,40 @@ class FirTypeResolverImpl(private val session: FirSession) : FirTypeResolver() {
         }
     }
 
+    @OptIn(SymbolInternals::class)
+    private fun FirQualifierResolver.resolveEnumEntrySymbol(
+        qualifier: List<FirQualifierPart>,
+        classId: ClassId
+    ): FirVariableSymbol<FirEnumEntry>? {
+        // Assuming the current qualifier refers to an enum entry, we drop the last part so we get a reference to the enum class.
+        val enumClassSymbol = resolveSymbolWithPrefix(qualifier.dropLast(1), classId) ?: return null
+        val enumClassFir = enumClassSymbol.fir as? FirRegularClass ?: return null
+        if (!enumClassFir.isEnumClass) return null
+        val enumEntryMatchingLastQualifier = enumClassFir.declarations
+            .firstOrNull { it is FirEnumEntry && it.name == qualifier.last().name } as? FirEnumEntry
+        return enumEntryMatchingLastQualifier?.symbol
+    }
+
+    @OptIn(SymbolInternals::class)
     private fun resolveUserType(
         typeRef: FirUserTypeRef,
-        symbol: FirClassifierSymbol<*>?,
+        symbol: FirBasedSymbol<*>?,
         substitutor: ConeSubstitutor?,
         areBareTypesAllowed: Boolean,
-        topDeclaration: FirRegularClass?
+        topDeclaration: FirRegularClass?,
+        isOperandOfIsOperator: Boolean
     ): ConeKotlinType {
-        if (symbol == null) {
+        if (symbol == null || symbol !is FirClassifierSymbol<*>) {
-            return ConeKotlinErrorType(ConeUnresolvedQualifierError(typeRef.render()))
+            val diagnostic = if (symbol?.fir is FirEnumEntry) {
+                if (isOperandOfIsOperator) {
+                    ConeSimpleDiagnostic("'is' operator can not be applied to an enum entry.", DiagnosticKind.IsEnumEntry)
+                } else {
+                    ConeSimpleDiagnostic("An enum entry should not be used as a type.", DiagnosticKind.EnumEntryAsType)
+                }
+            } else {
+                ConeUnresolvedQualifierError(typeRef.render())
+            }
+            return ConeKotlinErrorType(diagnostic)
         }
         if (symbol is FirTypeParameterSymbol) {
             for (part in typeRef.qualifier) {
@@ -191,6 +226,7 @@ class FirTypeResolverImpl(private val session: FirSession) : FirTypeResolver() {
             }
     }
 
+    @OptIn(SymbolInternals::class)
     private fun getClassesAlignedToQualifierParts(
         symbol: FirRegularClassSymbol,
         qualifier: List<FirQualifierPart>,
@@ -243,6 +279,7 @@ class FirTypeResolverImpl(private val session: FirSession) : FirTypeResolver() {
         val index: Int
     )
 
+    @OptIn(SymbolInternals::class)
     private fun createDiagnosticsIfExists(
         parameterClass: FirRegularClass?,
         qualifierPartIndex: Int,
@@ -314,13 +351,21 @@ class FirTypeResolverImpl(private val session: FirSession) : FirTypeResolver() {
     override fun resolveType(
         typeRef: FirTypeRef,
         scopeClassDeclaration: ScopeClassDeclaration,
-        areBareTypesAllowed: Boolean
+        areBareTypesAllowed: Boolean,
+        isOperandOfIsOperator: Boolean
     ): ConeKotlinType {
         return when (typeRef) {
             is FirResolvedTypeRef -> typeRef.type
             is FirUserTypeRef -> {
                 val (symbol, substitutor) = resolveToSymbol(typeRef, scopeClassDeclaration.scope)
-                resolveUserType(typeRef, symbol, substitutor, areBareTypesAllowed, scopeClassDeclaration.topDeclaration)
+                resolveUserType(
+                    typeRef,
+                    symbol,
+                    substitutor,
+                    areBareTypesAllowed,
+                    scopeClassDeclaration.topDeclaration,
+                    isOperandOfIsOperator
+                )
             }
             is FirFunctionTypeRef -> createFunctionalType(typeRef)
             is FirDynamicTypeRef -> ConeKotlinErrorType(ConeUnsupportedDynamicType())

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/FirSpecificTypeResolverTransformer.kt

@@ -36,6 +36,20 @@ class FirSpecificTypeResolverTransformer(
         }
     }
 
+    @set:PrivateForInline
+    var isOperandOfIsOperator: Boolean = false
+
+    @OptIn(PrivateForInline::class)
+    inline fun <R> withIsOperandOfIsOperator(block: () -> R): R {
+        val oldValue = isOperandOfIsOperator
+        isOperandOfIsOperator = true
+        return try {
+            block()
+        } finally {
+            isOperandOfIsOperator = oldValue
+        }
+    }
+
     @PrivateForInline
     @JvmField
     var currentFile: FirFile? = null
@@ -55,7 +69,7 @@ class FirSpecificTypeResolverTransformer(
     override fun transformTypeRef(typeRef: FirTypeRef, data: ScopeClassDeclaration): FirResolvedTypeRef {
         session.lookupTracker?.recordTypeLookup(typeRef, data.scope.scopeOwnerLookupNames, currentFile?.source)
         typeRef.transformChildren(this, data)
-        return transformType(typeRef, typeResolver.resolveType(typeRef, data, areBareTypesAllowed))
+        return transformType(typeRef, typeResolver.resolveType(typeRef, data, areBareTypesAllowed, isOperandOfIsOperator))
     }
 
     @OptIn(PrivateForInline::class)
@@ -65,7 +79,7 @@ class FirSpecificTypeResolverTransformer(
     ): FirResolvedTypeRef {
         functionTypeRef.transformChildren(this, data)
         session.lookupTracker?.recordTypeLookup(functionTypeRef, data.scope.scopeOwnerLookupNames, currentFile?.source)
-        val resolvedType = typeResolver.resolveType(functionTypeRef, data, areBareTypesAllowed).takeIfAcceptable()
+        val resolvedType = typeResolver.resolveType(functionTypeRef, data, areBareTypesAllowed, isOperandOfIsOperator).takeIfAcceptable()
         return if (resolvedType != null && resolvedType !is ConeClassErrorType) {
             buildResolvedTypeRef {
                 source = functionTypeRef.source

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/body/resolve/FirExpressionsResolveTransformer.kt

@@ -606,7 +606,13 @@ open class FirExpressionsResolveTransformer(transformer: FirBodyResolveTransform
         data: ResolutionMode,
     ): FirStatement {
         val resolved = components.typeResolverTransformer.withAllowedBareTypes {
-            typeOperatorCall.transformConversionTypeRef(transformer, ResolutionMode.ContextIndependent)
+            if (typeOperatorCall.operation == FirOperation.IS || typeOperatorCall.operation == FirOperation.NOT_IS) {
+                components.typeResolverTransformer.withIsOperandOfIsOperator {
+                    typeOperatorCall.transformConversionTypeRef(transformer, ResolutionMode.ContextIndependent)
+                }
+            } else {
+                typeOperatorCall.transformConversionTypeRef(transformer, ResolutionMode.ContextIndependent)
+            }
         }.transformTypeOperatorCallChildren()
 
         val conversionTypeRef = resolved.conversionTypeRef.withTypeArgumentsForBareType(resolved.argument)

compiler/fir/tree/src/org/jetbrains/kotlin/fir/diagnostics/ConeSimpleDiagnostic.kt

@@ -48,5 +48,8 @@ enum class DiagnosticKind {
     FloatLiteralOutOfRange,
     WrongLongSuffix,
 
+    IsEnumEntry,
+    EnumEntryAsType,
+
     Other,
 }

compiler/testData/diagnostics/tests/EnumEntryAsType.fir.kt

@@ -6,49 +6,49 @@ enum class Color {
     }
 }
 
-class MyColor(val x: <!UNRESOLVED_REFERENCE!>Color.RED<!>, y: <!UNRESOLVED_REFERENCE!>Color.RED<!>) : <!UNRESOLVED_REFERENCE!>Color.RED<!> {
+class MyColor(val x: <!ENUM_ENTRY_AS_TYPE!>Color.RED<!>, y: <!ENUM_ENTRY_AS_TYPE!>Color.RED<!>) : <!ENUM_ENTRY_AS_TYPE!>Color.RED<!> {
 
-    var z: <!UNRESOLVED_REFERENCE!>Color.RED<!> = Color.RED
+    var z: <!ENUM_ENTRY_AS_TYPE!>Color.RED<!> = Color.RED
-    set(arg: <!UNRESOLVED_REFERENCE!>Color.RED<!>) { z = arg }
+    set(arg: <!ENUM_ENTRY_AS_TYPE!>Color.RED<!>) { z = arg }
 
-    fun foo(arg: <!UNRESOLVED_REFERENCE!>Color.RED<!>): <!UNRESOLVED_REFERENCE!>Color.RED<!> = arg
+    fun foo(arg: <!ENUM_ENTRY_AS_TYPE!>Color.RED<!>): <!ENUM_ENTRY_AS_TYPE!>Color.RED<!> = arg
 
-    fun bar(): <!UNRESOLVED_REFERENCE!>Color.RED<!> {
+    fun bar(): <!ENUM_ENTRY_AS_TYPE!>Color.RED<!> {
-        class Local : <!UNRESOLVED_REFERENCE!>Color.RED<!>
+        class Local : <!ENUM_ENTRY_AS_TYPE!>Color.RED<!>
-        fun local(arg: <!UNRESOLVED_REFERENCE!>Color.RED<!>): <!UNRESOLVED_REFERENCE!>Color.RED<!> = arg
+        fun local(arg: <!ENUM_ENTRY_AS_TYPE!>Color.RED<!>): <!ENUM_ENTRY_AS_TYPE!>Color.RED<!> = arg
-        val temp: <!UNRESOLVED_REFERENCE!>Color.RED<!> = Color.RED
+        val temp: <!ENUM_ENTRY_AS_TYPE!>Color.RED<!> = Color.RED
-        temp as? <!UNRESOLVED_REFERENCE, UNRESOLVED_REFERENCE!>Color.RED<!>
+        temp as? <!ENUM_ENTRY_AS_TYPE, ENUM_ENTRY_AS_TYPE!>Color.RED<!>
-        if (temp is <!UNRESOLVED_REFERENCE!>Color.RED<!>) {
+        if (temp is <!IS_ENUM_ENTRY!>Color.RED<!>) {
-        return temp as <!UNRESOLVED_REFERENCE!>Color.RED<!>
+        return temp as <!ENUM_ENTRY_AS_TYPE!>Color.RED<!>
     }
-        val obj = object : <!UNRESOLVED_REFERENCE!>Color.RED<!> {}
+        val obj = object : <!ENUM_ENTRY_AS_TYPE!>Color.RED<!> {}
-        if (obj is <!UNRESOLVED_REFERENCE!>Color.RED<!>) {
+        if (obj is <!IS_ENUM_ENTRY!>Color.RED<!>) {
         return obj
     }
         return Color.RED
     }
 }
 
-fun create(): Array<<!UNRESOLVED_REFERENCE!>Color.RED<!>>? = null
+fun create(): Array<<!ENUM_ENTRY_AS_TYPE!>Color.RED<!>>? = null
 
-interface Your<T : <!UNRESOLVED_REFERENCE!>Color.RED<!>>
+interface Your<T : <!ENUM_ENTRY_AS_TYPE!>Color.RED<!>>
 
-class His : Your<<!UNRESOLVED_REFERENCE!>Color.RED<!>>
+class His : Your<<!ENUM_ENTRY_AS_TYPE!>Color.RED<!>>
 
-fun <T : <!UNRESOLVED_REFERENCE!>Color.RED<!>> otherCreate(): Array<T>? = null
+fun <T : <!ENUM_ENTRY_AS_TYPE!>Color.RED<!>> otherCreate(): Array<T>? = null
 
-typealias RedAlias = <!UNRESOLVED_REFERENCE!>Color.RED<!>
+typealias RedAlias = <!ENUM_ENTRY_AS_TYPE!>Color.RED<!>
 
-typealias ArrayOfEnumEntry = Array<<!UNRESOLVED_REFERENCE!>Color.RED<!>>
+typealias ArrayOfEnumEntry = Array<<!ENUM_ENTRY_AS_TYPE!>Color.RED<!>>
 
 typealias ArrayOfEnumEntryAlias = Array<RedAlias>
 
 fun <T> bar(a: Any): T = a as T
 
 fun <T> foo() {
-    foo<<!UNRESOLVED_REFERENCE!>Color.RED<!>>()
+    foo<<!ENUM_ENTRY_AS_TYPE!>Color.RED<!>>()
     foo<<!CANNOT_INFER_PARAMETER_TYPE!>RedAlias<!>>()
-    bar<<!UNRESOLVED_REFERENCE!>Color.RED<!>>(<!ARGUMENT_TYPE_MISMATCH!>Color.RED<!>)
+    bar<<!ENUM_ENTRY_AS_TYPE!>Color.RED<!>>(<!ARGUMENT_TYPE_MISMATCH!>Color.RED<!>)
 }
 
-fun Array<<!UNRESOLVED_REFERENCE!>Color.RED<!>>.foo(entries: Array<<!UNRESOLVED_REFERENCE!>Color.RED<!>>): Array<<!UNRESOLVED_REFERENCE!>Color.RED<!>> = null!!
+fun Array<<!ENUM_ENTRY_AS_TYPE!>Color.RED<!>>.foo(entries: Array<<!ENUM_ENTRY_AS_TYPE!>Color.RED<!>>): Array<<!ENUM_ENTRY_AS_TYPE!>Color.RED<!>> = null!!

compiler/testData/diagnostics/tests/enum/ifEnumEntry.fir.kt

@@ -3,4 +3,4 @@ enum class MyEnum {
     SECOND
 }
 
-fun foo(me: MyEnum): Boolean = if (me is <!UNRESOLVED_REFERENCE!>MyEnum.FIRST<!>) true else false
+fun foo(me: MyEnum): Boolean = if (me is <!IS_ENUM_ENTRY!>MyEnum.FIRST<!>) true else false

compiler/testData/diagnostics/tests/enum/inheritFromEnumEntry.fir.kt

@@ -2,4 +2,4 @@ enum class E {
     ENTRY
 }
 
-class A : <!UNRESOLVED_REFERENCE!>E.ENTRY<!>
+class A : <!ENUM_ENTRY_AS_TYPE!>E.ENTRY<!>

compiler/testData/diagnostics/tests/enum/isEnumEntry.fir.kt

@@ -3,4 +3,4 @@ enum class MyEnum {
     SECOND
 }
 
-fun foo(me: MyEnum): Boolean = me is <!UNRESOLVED_REFERENCE!>MyEnum.FIRST<!>
+fun foo(me: MyEnum): Boolean = me is <!IS_ENUM_ENTRY!>MyEnum.FIRST<!>

compiler/testData/diagnostics/tests/when/ExhaustiveEnumIs.fir.kt

@@ -13,8 +13,8 @@ enum class MyEnum {
 
 fun foo(x: MyEnum): Int {
     return <!NO_ELSE_IN_WHEN!>when<!> (x) {
-        is <!UNRESOLVED_REFERENCE!>MyEnum.A<!> -> 1
+        is <!IS_ENUM_ENTRY!>MyEnum.A<!> -> 1
-        is <!UNRESOLVED_REFERENCE!>MyEnum.B<!> -> 2
+        is <!IS_ENUM_ENTRY!>MyEnum.B<!> -> 2
-        is <!UNRESOLVED_REFERENCE!>MyEnum.C<!> -> 3
+        is <!IS_ENUM_ENTRY!>MyEnum.C<!> -> 3
     }
 }

compiler/testData/diagnostics/tests/when/ExhaustiveEnumMixed.fir.kt

@@ -15,7 +15,7 @@ enum class MyEnum {
 fun foo(x: MyEnum): Int {
     return <!NO_ELSE_IN_WHEN!>when<!> (x) {
         MyEnum.A -> 1
-        is <!UNRESOLVED_REFERENCE!>MyEnum.B<!> -> 2
+        is <!IS_ENUM_ENTRY!>MyEnum.B<!> -> 2
-        is <!UNRESOLVED_REFERENCE!>MyEnum.C<!> -> 3
+        is <!IS_ENUM_ENTRY!>MyEnum.C<!> -> 3
     }
 }

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDataClassConverters.kt

@@ -2616,6 +2616,18 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.IS_ENUM_ENTRY) { firDiagnostic ->
+        IsEnumEntryImpl(
+            firDiagnostic as FirPsiDiagnostic,
+            token,
+        )
+    }
+    add(FirErrors.ENUM_ENTRY_AS_TYPE) { firDiagnostic ->
+        EnumEntryAsTypeImpl(
+            firDiagnostic as FirPsiDiagnostic,
+            token,
+        )
+    }
     add(FirErrors.EXPECTED_CONDITION) { firDiagnostic ->
         ExpectedConditionImpl(
             firDiagnostic as FirPsiDiagnostic,

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnostics.kt

@@ -1834,6 +1834,14 @@ sealed class KtFirDiagnostic<PSI : PsiElement> : KtDiagnosticWithPsi<PSI> {
         abstract val compileTimeCheckResult: Boolean
     }
 
+    abstract class IsEnumEntry : KtFirDiagnostic<KtTypeReference>() {
+        override val diagnosticClass get() = IsEnumEntry::class
+    }
+
+    abstract class EnumEntryAsType : KtFirDiagnostic<KtTypeReference>() {
+        override val diagnosticClass get() = EnumEntryAsType::class
+    }
+
     abstract class ExpectedCondition : KtFirDiagnostic<KtWhenCondition>() {
         override val diagnosticClass get() = ExpectedCondition::class
     }

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnosticsImpl.kt

@@ -2954,6 +2954,20 @@ internal class UselessIsCheckImpl(
     override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
 }
 
+internal class IsEnumEntryImpl(
+    firDiagnostic: FirPsiDiagnostic,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.IsEnumEntry(), KtAbstractFirDiagnostic<KtTypeReference> {
+    override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
+}
+
+internal class EnumEntryAsTypeImpl(
+    firDiagnostic: FirPsiDiagnostic,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.EnumEntryAsType(), KtAbstractFirDiagnostic<KtTypeReference> {
+    override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
+}
+
 internal class ExpectedConditionImpl(
     firDiagnostic: FirPsiDiagnostic,
     override val token: ValidityToken,