[FIR] Implement RETURN_TYPE_MISMATCH_BY_DELEGATION diagnostic

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

@@ -1654,9 +1654,16 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
     }
     add(FirErrors.RETURN_TYPE_MISMATCH_ON_INHERITANCE) { firDiagnostic ->
         ReturnTypeMismatchOnInheritanceImpl(
-            firSymbolBuilder.classifierBuilder.buildClassLikeSymbol(firDiagnostic.a),
+            firSymbolBuilder.callableBuilder.buildCallableSymbol(firDiagnostic.a as FirCallableDeclaration),
+            firSymbolBuilder.callableBuilder.buildCallableSymbol(firDiagnostic.b as FirCallableDeclaration),
+            firDiagnostic as FirPsiDiagnostic<*>,
+            token,
+        )
+    }
+    add(FirErrors.RETURN_TYPE_MISMATCH_BY_DELEGATION) { firDiagnostic ->
+        ReturnTypeMismatchByDelegationImpl(
+            firSymbolBuilder.callableBuilder.buildCallableSymbol(firDiagnostic.a as FirCallableDeclaration),
             firSymbolBuilder.callableBuilder.buildCallableSymbol(firDiagnostic.b as FirCallableDeclaration),
-            firSymbolBuilder.callableBuilder.buildCallableSymbol(firDiagnostic.c as FirCallableDeclaration),
             firDiagnostic as FirPsiDiagnostic<*>,
             token,
         )

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

@@ -1173,11 +1173,16 @@ sealed class KtFirDiagnostic<PSI: PsiElement> : KtDiagnosticWithPsi<PSI> {
 
     abstract class ReturnTypeMismatchOnInheritance : KtFirDiagnostic<KtClassOrObject>() {
         override val diagnosticClass get() = ReturnTypeMismatchOnInheritance::class
-        abstract val classOrObject: KtClassLikeSymbol
         abstract val conflictingDeclaration1: KtCallableSymbol
         abstract val conflictingDeclaration2: KtCallableSymbol
     }
 
+    abstract class ReturnTypeMismatchByDelegation : KtFirDiagnostic<KtClassOrObject>() {
+        override val diagnosticClass get() = ReturnTypeMismatchByDelegation::class
+        abstract val delegateDeclaration: KtCallableSymbol
+        abstract val baseDeclaration: KtCallableSymbol
+    }
+
     abstract class AbstractMemberNotImplemented : KtFirDiagnostic<KtClassOrObject>() {
         override val diagnosticClass get() = AbstractMemberNotImplemented::class
         abstract val classOrObject: KtClassLikeSymbol

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

@@ -1884,7 +1884,6 @@ internal class OverridingFinalMemberImpl(
 }
 
 internal class ReturnTypeMismatchOnInheritanceImpl(
-    override val classOrObject: KtClassLikeSymbol,
     override val conflictingDeclaration1: KtCallableSymbol,
     override val conflictingDeclaration2: KtCallableSymbol,
     firDiagnostic: FirPsiDiagnostic<*>,
@@ -1893,6 +1892,15 @@ internal class ReturnTypeMismatchOnInheritanceImpl(
     override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
 }
 
+internal class ReturnTypeMismatchByDelegationImpl(
+    override val delegateDeclaration: KtCallableSymbol,
+    override val baseDeclaration: KtCallableSymbol,
+    firDiagnostic: FirPsiDiagnostic<*>,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.ReturnTypeMismatchByDelegation(), KtAbstractFirDiagnostic<KtClassOrObject> {
+    override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
+}
+
 internal class AbstractMemberNotImplementedImpl(
     override val classOrObject: KtClassLikeSymbol,
     override val missingDeclaration: KtCallableSymbol,