java completion: prefer lambdas over method references (IDEA-181662)

java/java-impl/src/com/intellij/codeInsight/completion/JavaCompletionSorting.java

@@ -457,7 +457,7 @@ public class JavaCompletionSorting {
     @Override
     public Comparable weigh(@NotNull LookupElement element) {
       final Object object = element.getObject();
-      if (object instanceof PsiMethod) {
+      if (object instanceof PsiMethod && FunctionalExpressionCompletionProvider.FUNCTIONAL_EXPR_ITEM.get(element) == null) {
         PsiType type = ((PsiMethod)object).getReturnType();
         final JavaMethodCallElement callItem = element.as(JavaMethodCallElement.CLASS_CONDITION_KEY);
         if (callItem != null) {

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/lambda/completion/normal/PreferLambdaToConstructorReference.java

@@ -0,0 +1,7 @@
+import java.util.*;
+
+class Foo {
+  {
+    Optional.of(2).orElseThrow(<caret>)
+  }
+}

java/java-tests/testSrc/com/intellij/java/codeInsight/completion/Normal8CompletionTest.groovy

@@ -306,6 +306,11 @@ class Test88 {
     myFixture.assertPreferredCompletionItems 0, 'output', 'out -> '
   }
 
+  void testPreferLambdaToConstructorReference() {
+    configureByTestName()
+    myFixture.assertPreferredCompletionItems 0, '() -> ', 'AbstractMethodError::new'
+  }
+
   private checkResultByFileName() {
     checkResultByFile(getTestName(false) + "_after.java")
   }

java/java-tests/testSrc/com/intellij/java/codeInsight/completion/SmartType18CompletionTest.java

@@ -110,7 +110,8 @@ public class SmartType18CompletionTest extends LightFixtureCompletionTestCase {
     myFixture.addClass("package foo; class ImplInaccessible implements intf.Intf<String> {}");
 
     configureByTestName();
-    myFixture.assertPreferredCompletionItems(0, "ImplBar::new", "ImplFoo::new", "() -> ");
+    myFixture.assertPreferredCompletionItems(0, "() -> ", "ImplBar::new", "ImplFoo::new");
+    myFixture.getLookup().setCurrentItem(myFixture.getLookupElements()[1]);
     myFixture.type('\n');
     checkResultByFile("/" + getTestName(false) + "-out.java");
   }