Correct handling of empty super type list while adding an entry #KT-12011 Fixed

compiler/frontend/src/org/jetbrains/kotlin/psi/KtClassOrObject.kt

@@ -38,6 +38,10 @@ abstract class KtClassOrObject :
 
     fun addSuperTypeListEntry(superTypeListEntry: KtSuperTypeListEntry): KtSuperTypeListEntry {
         getSuperTypeList()?.let {
+            val single = it.entries.singleOrNull()
+            if (single != null && single.typeReference?.typeElement == null) {
+                return single.replace(superTypeListEntry) as KtSuperTypeListEntry
+            }
             return EditCommaSeparatedListHelper.addItem(it, getSuperTypeListEntries(), superTypeListEntry)
         }
 

idea/testData/quickfix/typeMismatch/letObjectLiteralImplementInterface.kt

@@ -0,0 +1,9 @@
+// "Let the anonymous object implement interface 'Runnable'" "true"
+
+fun foo(r: Runnable) {}
+
+fun bar() {
+    foo(<caret>object: {})
+}
+
+interface Runnable

idea/testData/quickfix/typeMismatch/letObjectLiteralImplementInterface.kt.after

@@ -0,0 +1,9 @@
+// "Let the anonymous object implement interface 'Runnable'" "true"
+
+fun foo(r: Runnable) {}
+
+fun bar() {
+    foo(object: Runnable {})
+}
+
+interface Runnable

idea/tests/org/jetbrains/kotlin/idea/quickfix/QuickFixTestGenerated.java

@@ -7654,6 +7654,12 @@ public class QuickFixTestGenerated extends AbstractQuickFixTest {
             doTest(fileName);
         }
 
+        @TestMetadata("letObjectLiteralImplementInterface.kt")
+        public void testLetObjectLiteralImplementInterface() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("idea/testData/quickfix/typeMismatch/letObjectLiteralImplementInterface.kt");
+            doTest(fileName);
+        }
+
         @TestMetadata("letStringImplementInterface.kt")
         public void testLetStringImplementInterface() throws Exception {
             String fileName = KotlinTestUtils.navigationMetadata("idea/testData/quickfix/typeMismatch/letStringImplementInterface.kt");