[FIR] Add proper nullability for java enums

compiler/fir/java/src/org/jetbrains/kotlin/fir/java/JavaSymbolProvider.kt

@@ -16,6 +16,7 @@ import org.jetbrains.kotlin.fir.declarations.FirRegularClass
 import org.jetbrains.kotlin.fir.declarations.FirTypeParameter
 import org.jetbrains.kotlin.fir.declarations.addDefaultBoundIfNecessary
 import org.jetbrains.kotlin.fir.declarations.builder.FirTypeParameterBuilder
+import org.jetbrains.kotlin.fir.expressions.builder.buildAnnotationCall
 import org.jetbrains.kotlin.fir.java.declarations.*
 import org.jetbrains.kotlin.fir.resolve.AbstractFirSymbolProvider
 import org.jetbrains.kotlin.fir.resolve.constructType
@@ -194,6 +195,7 @@ class JavaSymbolProvider(
                             returnTypeRef = returnType.toFirJavaTypeRef(this@JavaSymbolProvider.session, javaTypeParameterStack)
                             isVar = !javaField.isFinal
                             isStatic = javaField.isStatic
+                            isEnumEntry = javaField.isEnumEntry
                             addAnnotationsFrom(this@JavaSymbolProvider.session, javaField, javaTypeParameterStack)
                         }
                         declarations += firJavaField

compiler/fir/java/src/org/jetbrains/kotlin/fir/java/declarations/FirJavaField.kt

@@ -42,6 +42,7 @@ class FirJavaField @FirImplementationDetail constructor(
     override val isVar: Boolean,
     override val annotations: MutableList<FirAnnotationCall>,
     override val typeParameters: MutableList<FirTypeParameter>,
+    val isEnumEntry: Boolean
 ) : FirField() {
     init {
         symbol.bind(this)
@@ -127,6 +128,7 @@ internal class FirJavaFieldBuilder : FirFieldBuilder() {
     var modality: Modality? = null
     lateinit var visibility: Visibility
     var isStatic: Boolean by Delegates.notNull()
+    var isEnumEntry: Boolean by Delegates.notNull()
 
     override var resolvePhase: FirResolvePhase = FirResolvePhase.ANALYZED_DEPENDENCIES
 
@@ -149,7 +151,8 @@ internal class FirJavaFieldBuilder : FirFieldBuilder() {
             status,
             isVar,
             annotations,
-            typeParameters
+            typeParameters,
+            isEnumEntry
         )
     }
 }

compiler/fir/java/src/org/jetbrains/kotlin/fir/java/scopes/JavaClassEnhancementScope.kt

@@ -33,8 +33,7 @@ import org.jetbrains.kotlin.fir.types.jvm.FirJavaTypeRef
 import org.jetbrains.kotlin.load.java.AnnotationTypeQualifierResolver
 import org.jetbrains.kotlin.load.java.descriptors.NullDefaultValue
 import org.jetbrains.kotlin.load.java.descriptors.StringDefaultValue
-import org.jetbrains.kotlin.load.java.typeEnhancement.PREDEFINED_FUNCTION_ENHANCEMENT_INFO_BY_SIGNATURE
-import org.jetbrains.kotlin.load.java.typeEnhancement.PredefinedFunctionEnhancementInfo
+import org.jetbrains.kotlin.load.java.typeEnhancement.*
 import org.jetbrains.kotlin.load.kotlin.SignatureBuildingComponents
 import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.utils.Jsr305State
@@ -89,7 +88,18 @@ class JavaClassEnhancementScope(
             is FirField -> {
                 if (firElement.returnTypeRef !is FirJavaTypeRef) return original
                 val memberContext = context.copyWithNewDefaultTypeQualifiers(typeQualifierResolver, jsr305State, firElement.annotations)
-                val newReturnTypeRef = enhanceReturnType(firElement, emptyList(), memberContext, null)
+                val isEnumEntry = (firElement as? FirJavaField)?.isEnumEntry ?: false
+
+                val predefinedInfo = if (isEnumEntry) {
+                    PredefinedFunctionEnhancementInfo(
+                        TypeEnhancementInfo(0 to JavaTypeQualifiers(NullabilityQualifier.NOT_NULL, null, false)),
+                        emptyList()
+                    )
+                } else {
+                    null
+                }
+
+                val newReturnTypeRef = enhanceReturnType(firElement, emptyList(), memberContext, predefinedInfo)
 
                 val symbol = FirFieldSymbol(original.callableId)
                 buildJavaField {
@@ -103,6 +113,7 @@ class JavaClassEnhancementScope(
                     isVar = firElement.isVar
                     isStatic = firElement.isStatic
                     annotations += firElement.annotations
+                    this.isEnumEntry = isEnumEntry
                 }
                 return symbol
             }