method reference: use site substitutor if nothing else was inferred

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

@@ -734,6 +734,7 @@ public class InferenceSession {
         substitutor = substitutor.put(typeParameter, runtimeException);
       } 
       else {
+        if (substitutor.getSubstitutionMap().get(typeParameter) != null) continue;
         substitutor = substitutor.put(typeParameter, myErased ? null : getUpperBound(var, substitutor));
       }
     }

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/lambda/newMethodRef/IDEA122018comment.java

@@ -0,0 +1,20 @@
+import java.util.List;
+import java.util.function.Function;
+class Base {
+
+  interface Seq<Eq> extends Iterable<Eq> {
+    static <E> Seq<E> of(Iterable<? extends E> source) {
+      return null;
+    }
+
+    <R> Seq<R> map(Function<? super Eq, ? extends R> mapper);
+  }
+
+}
+
+class Test3 extends Base {
+
+  void test4(Seq<List<String>> map) {
+    Seq<Seq<String>> mapped = map.map(Seq::of);
+  }
+}

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

@@ -213,6 +213,10 @@ public class NewMethodRefHighlightingTest extends LightDaemonAnalyzerTestCase {
     doTest();
   }
 
+  public void testIDEA122018comment() throws Exception {
+    doTest();
+  }
+
   private void doTest() {
     doTest(false);
   }