method refs: infer 'diamond' if constructor ref type is raw (IDEA-142142)

java/java-psi-impl/src/com/intellij/psi/impl/source/resolve/graphInference/constraints/PsiMethodReferenceCompatibilityConstraint.java

@@ -227,6 +227,19 @@ public class PsiMethodReferenceCompatibilityConstraint implements ConstraintForm
           psiSubstitutor = TypeConversionUtil.getClassSubstitutor(qContainingClass, paramClass, resolveResult.getSubstitutor());
           LOG.assertTrue(psiSubstitutor != null);
         }
+        else if (member instanceof PsiMethod && ((PsiMethod)member).isConstructor() || member instanceof PsiClass) {
+          //15.13.1 
+          //If ClassType is a raw type, but is not a non-static member type of a raw type, 
+          //the candidate notional member methods are those specified in §15.9.3 for a class instance creation expression that uses <> 
+          //to elide the type arguments to a class.
+          final PsiResolveHelper helper = JavaPsiFacade.getInstance(myExpression.getProject()).getResolveHelper();
+          final PsiType[] paramTypes =
+            member instanceof PsiMethod ? ((PsiMethod)member).getSignature(PsiSubstitutor.EMPTY).getParameterTypes() : PsiType.EMPTY_ARRAY;
+          psiSubstitutor = helper.inferTypeArguments(qContainingClass.getTypeParameters(),
+                                                     paramTypes,
+                                                     signature.getParameterTypes(),
+                                                     PsiUtil.getLanguageLevel(myExpression));
+        }
         else {
           psiSubstitutor = PsiSubstitutor.EMPTY;
         }

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/lambda/newMethodRef/RawClassTypeOnConstructorReference.java

@@ -0,0 +1,6 @@
+import java.util.*;
+class Test {
+  public Optional<Set<String>> foo(Optional<Set<String>> args) {
+     return args.map(HashSet::new).map(Collections::unmodifiableSet);
+  }
+}

java/java-tests/testSrc/com/intellij/codeInsight/daemon/lambda/NewMethodRefHighlightingTest.java

@@ -414,6 +414,10 @@ public class NewMethodRefHighlightingTest extends LightDaemonAnalyzerTestCase {
     doTest();
   }
 
+  public void testRawClassTypeOnConstructorReference() throws Exception {
+    doTest();
+  }
+
   private void doTest() {
     doTest(false);
   }