don't erase method invocation type iff unchecked conversion was required during additional constraints processing (IDEA-163503)

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

@@ -86,6 +86,7 @@ public class InferenceSession {
   private List<String> myErrorMessages;
   
   private boolean myErased;
+  private boolean myCheckApplicabilityPhase = true;
 
   public final InferenceIncorporationPhase myIncorporationPhase = new InferenceIncorporationPhase(this);
 
@@ -374,6 +375,7 @@ public class InferenceSession {
       return;
     }
 
+    myCheckApplicabilityPhase = false;
     if (properties != null && !properties.isApplicabilityCheck()) {
       final PsiMethod method = properties.getMethod();
       if (parent instanceof PsiCallExpression && PsiPolyExpressionUtil.isMethodCallPolyExpression((PsiExpression)parent, method)) {
@@ -1669,6 +1671,12 @@ public class InferenceSession {
     return null;
   }
 
+  public void setErasedDuringApplicabilityCheck() {
+    if (myCheckApplicabilityPhase) {
+      myErased = true;
+    }
+  }
+
   public void setErased() {
     myErased = true;
   }

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2000-2013 JetBrains s.r.o.
+ * Copyright 2000-2016 JetBrains s.r.o.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -61,7 +61,7 @@ public class TypeCompatibilityConstraint implements ConstraintFormula {
     }
 
     if (isUncheckedConversion(myT, myS)) {
-      session.setErased();
+      session.setErasedDuringApplicabilityCheck();
       return true;
     }
 

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/lambda/graphInference/ErasureOfReturnTypeIffUncheckedConversionWasNecessaryDuringApplicabilityCheckOnly.java

@@ -0,0 +1,8 @@
+import java.util.List;
+import java.util.stream.Collectors;
+
+class Test {
+  private static List<Object> test(List<List> list) {
+    return list.stream().flatMap(List::stream).collect(Collectors.toList());
+  }
+}

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2000-2014 JetBrains s.r.o.
+ * Copyright 2000-2016 JetBrains s.r.o.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -18,16 +18,13 @@ package com.intellij.codeInsight.daemon.lambda;
 import com.intellij.codeInsight.daemon.LightDaemonAnalyzerTestCase;
 import com.intellij.codeInsight.daemon.impl.HighlightInfo;
 import com.intellij.codeInspection.uncheckedWarnings.UncheckedWarningLocalInspection;
-import com.intellij.idea.Bombed;
 import com.intellij.lang.annotation.HighlightSeverity;
 import com.intellij.openapi.projectRoots.JavaSdkVersion;
-import com.intellij.openapi.projectRoots.Sdk;
 import com.intellij.openapi.util.text.StringUtil;
 import com.intellij.testFramework.IdeaTestUtil;
 import org.jetbrains.annotations.NonNls;
 
 import java.util.ArrayList;
-import java.util.Calendar;
 import java.util.Collection;
 import java.util.List;
 
@@ -486,6 +483,10 @@ public class GraphInferenceHighlightingTest extends LightDaemonAnalyzerTestCase
     doTest();
   }
 
+  public void testErasureOfReturnTypeIffUncheckedConversionWasNecessaryDuringApplicabilityCheckOnly() throws Exception {
+    doTest();
+  }
+
   private void doTest() throws Exception {
     doTest(false);
   }