generics: do not prefer interface over super class even if return type is more specific (IDEA-110568)

java/java-psi-impl/src/com/intellij/psi/scope/conflictResolvers/JavaMethodsConflictResolver.java

@@ -302,6 +302,7 @@ public class JavaMethodsConflictResolver implements PsiConflictResolver{
               final PsiType returnType = eSubstitutor.substitute(existingMethod.getReturnType());
               final PsiType returnType1 = cSubstitutor.substitute(method.getReturnType());
               if (returnType != null && returnType1 != null && !returnType1.equals(returnType) && TypeConversionUtil.isAssignable(returnType, returnType1, false)) {
+                if (class1.isInterface() && !existingClass.isInterface()) continue;
                 conflicts.remove(existing);
               } else {
                 conflicts.remove(i);

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/genericsHighlighting/IDEA110568.java

@@ -0,0 +1,20 @@
+import java.util.Collection;
+
+public class IncorrectError extends NarrowClass {
+    public Collection<String> bar() {
+        return super.doStuff();
+    }
+}
+
+interface Interface {
+    Collection<String> doStuff();
+}
+
+class NarrowClass extends BaseClass implements Interface {
+}
+
+class BaseClass {
+    public Collection doStuff() {
+        return null;
+    }
+}

java/java-tests/testSrc/com/intellij/codeInsight/daemon/GenericsHighlightingTest.java

@@ -244,6 +244,7 @@ public class GenericsHighlightingTest extends LightDaemonAnalyzerTestCase {
   public void testIllegalForwardReferenceInTypeParameterDefinition() throws Exception { doTest5(false);}
 
   public void testIDEA57877() throws Exception { doTest5(false);}
+  public void testIDEA110568() throws Exception { doTest5(false);}
   public void testTypeParamsCyclicInference() throws Exception { doTest5(false);}
   public void testCaptureTopLevelWildcardsForConditionalExpression() throws Exception { doTest5(false);}
   public void testGenericsOverrideMethodInRawInheritor() throws Exception { doTest5(false);}