[FE] Prohibit sealed fun interfaces

#KT-44947 Fixed

compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirOldFrontendDiagnosticsTestGenerated.java

@@ -24496,6 +24496,12 @@ public class FirOldFrontendDiagnosticsTestGenerated extends AbstractFirDiagnosti
                     runTest("compiler/testData/diagnostics/tests/sealed/interfaces/inheritorInDifferentModule.kt");
                 }
 
+                @Test
+                @TestMetadata("sealedFunInterface.kt")
+                public void testSealedFunInterface() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/sealed/interfaces/sealedFunInterface.kt");
+                }
+
                 @Test
                 @TestMetadata("sealedInterfacesDisabled.kt")
                 public void testSealedInterfacesDisabled() throws Exception {

compiler/frontend/src/org/jetbrains/kotlin/resolve/checkers/SealedInterfaceAllowedChecker.kt

@@ -14,10 +14,16 @@ import org.jetbrains.kotlin.psi.KtDeclaration
 
 object SealedInterfaceAllowedChecker : DeclarationChecker {
     override fun check(declaration: KtDeclaration, descriptor: DeclarationDescriptor, context: DeclarationCheckerContext) {
-        if (context.languageVersionSettings.supportsFeature(LanguageFeature.SealedInterfaces)) return
         if (descriptor !is ClassDescriptor) return
         if (descriptor.kind != ClassKind.INTERFACE) return
         val keyword = declaration.modifierList?.getModifier(KtTokens.SEALED_KEYWORD) ?: return
-        context.trace.report(Errors.UNSUPPORTED_FEATURE.on(keyword, LanguageFeature.SealedInterfaces to context.languageVersionSettings))
+        val diagnostic = if (context.languageVersionSettings.supportsFeature(LanguageFeature.SealedInterfaces)) {
+            if (descriptor.isFun) {
+                Errors.UNSUPPORTED.on(keyword, "sealed fun interfaces")
+            } else return
+        } else {
+            Errors.UNSUPPORTED_FEATURE.on(keyword, LanguageFeature.SealedInterfaces to context.languageVersionSettings)
+        }
+        context.trace.report(diagnostic)
     }
 }

compiler/testData/diagnostics/tests/sealed/interfaces/sealedFunInterface.fir.kt

@@ -0,0 +1,15 @@
+sealed fun interface A { // error
+    fun foo()
+}
+
+sealed interface Base {
+    sealed fun interface Derived : Base {  // error
+        fun foo()
+    }
+}
+
+sealed interface IBase {
+    fun interface IDerived : IBase { // OK
+        fun foo()
+    }
+}

compiler/testData/diagnostics/tests/sealed/interfaces/sealedFunInterface.kt

@@ -0,0 +1,15 @@
+<!UNSUPPORTED!>sealed<!> fun interface A { // error
+    fun foo()
+}
+
+sealed interface Base {
+    <!UNSUPPORTED!>sealed<!> fun interface Derived : Base {  // error
+        fun foo()
+    }
+}
+
+sealed interface IBase {
+    fun interface IDerived : IBase { // OK
+        fun foo()
+    }
+}

compiler/testData/diagnostics/tests/sealed/interfaces/sealedFunInterface.txt

@@ -0,0 +1,34 @@
+package
+
+public sealed fun interface A {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract fun foo(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public sealed interface Base {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+    public sealed fun interface Derived : Base {
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public abstract fun foo(): kotlin.Unit
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+}
+
+public sealed interface IBase {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+    public fun interface IDerived : IBase {
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public abstract fun foo(): kotlin.Unit
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+}

compiler/tests-common-new/tests-gen/org/jetbrains/kotlin/test/runners/DiagnosticTestGenerated.java

@@ -24586,6 +24586,12 @@ public class DiagnosticTestGenerated extends AbstractDiagnosticTest {
                     runTest("compiler/testData/diagnostics/tests/sealed/interfaces/inheritorInDifferentModule.kt");
                 }
 
+                @Test
+                @TestMetadata("sealedFunInterface.kt")
+                public void testSealedFunInterface() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/sealed/interfaces/sealedFunInterface.kt");
+                }
+
                 @Test
                 @TestMetadata("sealedInterfacesDisabled.kt")
                 public void testSealedInterfacesDisabled() throws Exception {