folded functional expression: compare unprocessed types (IDEA-194333)

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/quickFix/trivialMethodReference/afterGroundTypes.java

@@ -0,0 +1,11 @@
+// "Replace with qualifier" "true"
+import java.util.List;
+import java.util.function.Function;
+import java.util.stream.Stream;
+
+class Main {
+    void test(List<List<String>> list) {
+        Function<List<String>, Stream<String>> fn = List::stream;
+        System.out.println(list.stream().flatMap(fn).count());
+    }
+}

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/quickFix/trivialMethodReference/beforeGroundTypes.java

@@ -0,0 +1,11 @@
+// "Replace with qualifier" "true"
+import java.util.List;
+import java.util.function.Function;
+import java.util.stream.Stream;
+
+class Main {
+    void test(List<List<String>> list) {
+        Function<List<String>, Stream<String>> fn = List::stream;
+        System.out.println(list.stream().flatMap(fn::a<caret>pply).count());
+    }
+}

plugins/InspectionGadgets/src/com/intellij/codeInspection/FunctionalExpressionCanBeFoldedInspection.java

@@ -44,7 +44,8 @@ public class FunctionalExpressionCanBeFoldedInspection extends AbstractBaseJavaL
         if (qualifierExpression != null && referenceNameElement != null && !(qualifierExpression instanceof PsiSuperExpression)) {
           final PsiType qualifierType = qualifierExpression.getType();
           if (qualifierType != null) {
-            final PsiType functionalInterfaceType = expression.getFunctionalInterfaceType();
+            //don't get ground type as check is required over expected type instead
+            final PsiType functionalInterfaceType = LambdaUtil.getFunctionalInterfaceType(expression, true);
             final PsiMethod interfaceMethod = LambdaUtil.getFunctionalInterfaceMethod(functionalInterfaceType);
             if (interfaceMethod != null) {
               final PsiElement resolve = resolver.get();