[FIR] Implement RETURN_TYPE_MISMATCH_ON_INHERITANCE diagnostic

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

@@ -1652,6 +1652,15 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.RETURN_TYPE_MISMATCH_ON_INHERITANCE) { firDiagnostic ->
+        ReturnTypeMismatchOnInheritanceImpl(
+            firSymbolBuilder.classifierBuilder.buildClassLikeSymbol(firDiagnostic.a),
+            firSymbolBuilder.callableBuilder.buildCallableSymbol(firDiagnostic.b as FirCallableDeclaration),
+            firSymbolBuilder.callableBuilder.buildCallableSymbol(firDiagnostic.c as FirCallableDeclaration),
+            firDiagnostic as FirPsiDiagnostic<*>,
+            token,
+        )
+    }
     add(FirErrors.ABSTRACT_MEMBER_NOT_IMPLEMENTED) { firDiagnostic ->
         AbstractMemberNotImplementedImpl(
             firSymbolBuilder.classifierBuilder.buildClassLikeSymbol(firDiagnostic.a),

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

@@ -1171,6 +1171,13 @@ sealed class KtFirDiagnostic<PSI: PsiElement> : KtDiagnosticWithPsi<PSI> {
         abstract val containingClassName: Name
     }
 
+    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 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

@@ -1883,6 +1883,16 @@ internal class OverridingFinalMemberImpl(
     override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
 }
 
+internal class ReturnTypeMismatchOnInheritanceImpl(
+    override val classOrObject: KtClassLikeSymbol,
+    override val conflictingDeclaration1: KtCallableSymbol,
+    override val conflictingDeclaration2: KtCallableSymbol,
+    firDiagnostic: FirPsiDiagnostic<*>,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.ReturnTypeMismatchOnInheritance(), KtAbstractFirDiagnostic<KtClassOrObject> {
+    override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
+}
+
 internal class AbstractMemberNotImplementedImpl(
     override val classOrObject: KtClassLikeSymbol,
     override val missingDeclaration: KtCallableSymbol,