FIR: Render fun status flag in FirRenderer.

compiler/fir/tree/src/org/jetbrains/kotlin/fir/FirRenderer.kt

@@ -5,6 +5,7 @@
 
 package org.jetbrains.kotlin.fir
 
+import org.jetbrains.kotlin.descriptors.ClassKind
 import org.jetbrains.kotlin.descriptors.EffectiveVisibility
 import org.jetbrains.kotlin.descriptors.Visibilities
 import org.jetbrains.kotlin.descriptors.Visibility
@@ -324,7 +325,7 @@ class FirRenderer(builder: StringBuilder, private val mode: RenderMode = RenderM
             print("inner ")
         }
 
-        // `companion/data` modifiers are only valid for FirRegularClass, but we render them to make sure they are not
+        // `companion/data/fun` modifiers are only valid for FirRegularClass, but we render them to make sure they are not
         // incorrectly loaded for other declarations during deserialization.
         if (memberDeclaration.status.isCompanion) {
             print("companion ")
@@ -332,6 +333,13 @@ class FirRenderer(builder: StringBuilder, private val mode: RenderMode = RenderM
         if (memberDeclaration.status.isData) {
             print("data ")
         }
+        // All Java interfaces are considered `fun` (functional interfaces) for resolution purposes
+        // (see JavaSymbolProvider.createFirJavaClass). Don't render `fun` for Java interfaces; it's not a modifier in Java.
+        val isJavaInterface =
+            memberDeclaration is FirRegularClass && memberDeclaration.classKind == ClassKind.INTERFACE && memberDeclaration.isJava
+        if (memberDeclaration.status.isFun && !isJavaInterface) {
+            print("fun ")
+        }
 
         if (memberDeclaration.isInline) {
             print("inline ")

compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/FirDeclarationUtil.kt

@@ -12,7 +12,6 @@ import org.jetbrains.kotlin.fir.declarations.builder.FirTypeParameterBuilder
 import org.jetbrains.kotlin.fir.declarations.impl.FirFileImpl
 import org.jetbrains.kotlin.fir.declarations.impl.FirRegularClassImpl
 import org.jetbrains.kotlin.fir.expressions.FirQualifiedAccess
-import org.jetbrains.kotlin.fir.expressions.FirVariableAssignment
 import org.jetbrains.kotlin.fir.references.FirResolvedNamedReference
 import org.jetbrains.kotlin.fir.render
 import org.jetbrains.kotlin.fir.symbols.impl.FirAnonymousObjectSymbol
@@ -23,8 +22,6 @@ import org.jetbrains.kotlin.fir.types.ConeClassLikeType
 import org.jetbrains.kotlin.fir.types.ConeFlexibleType
 import org.jetbrains.kotlin.fir.types.builder.buildResolvedTypeRef
 import org.jetbrains.kotlin.fir.types.coneTypeSafe
-import org.jetbrains.kotlin.metadata.ProtoBuf
-import org.jetbrains.kotlin.metadata.deserialization.NameResolver
 
 fun FirTypeParameterBuilder.addDefaultBoundIfNecessary(isFlexible: Boolean = false) {
     if (bounds.isEmpty()) {
@@ -227,6 +224,8 @@ val FirQualifiedAccess.referredPropertySymbol: FirPropertySymbol?
         return reference.resolvedSymbol as? FirPropertySymbol
     }
 
+inline val FirDeclaration.isJava: Boolean
+    get() = origin == FirDeclarationOrigin.Java
 inline val FirDeclaration.isFromLibrary: Boolean
     get() = origin == FirDeclarationOrigin.Library
 inline val FirDeclaration.isSynthetic: Boolean