JS: prohibit external fun interface

compiler/testData/diagnostics/testsWithJsStdLib/native/externalFunInterface.kt

@@ -0,0 +1,3 @@
+external fun interface <!WRONG_EXTERNAL_DECLARATION!>I<!> {
+    fun f()
+}

compiler/testData/diagnostics/testsWithJsStdLib/native/externalFunInterface.txt

@@ -0,0 +1,8 @@
+package
+
+public external fun interface I {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract fun f(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}

compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestWithJsStdLibGenerated.java

@@ -681,6 +681,11 @@ public class DiagnosticsTestWithJsStdLibGenerated extends AbstractDiagnosticsTes
             runTest("compiler/testData/diagnostics/testsWithJsStdLib/native/extensionFunctionArgumentOrReturnType.kt");
         }
 
+        @TestMetadata("externalFunInterface.kt")
+        public void testExternalFunInterface() throws Exception {
+            runTest("compiler/testData/diagnostics/testsWithJsStdLib/native/externalFunInterface.kt");
+        }
+
         @TestMetadata("externalInterfaceNested.kt")
         public void testExternalInterfaceNested() throws Exception {
             runTest("compiler/testData/diagnostics/testsWithJsStdLib/native/externalInterfaceNested.kt");

js/js.frontend/src/org/jetbrains/kotlin/js/resolve/diagnostics/JsExternalChecker.kt

@@ -44,6 +44,7 @@ object JsExternalChecker : DeclarationChecker {
                 descriptor.isData -> "data class"
                 descriptor.isInner -> "inner class"
                 descriptor.isInline -> "inline class"
+                descriptor.isFun -> "fun interface"
                 DescriptorUtils.isAnnotationClass(descriptor) -> "annotation class"
                 else -> null
             }