java: single underscore variables highlighting for Java 8

java/java-impl/src/com/intellij/codeInsight/daemon/impl/analysis/HighlightUtil.java

@@ -622,6 +622,22 @@ public class HighlightUtil extends HighlightUtilBase {
     return null;
   }
 
+  @Nullable
+  public static HighlightInfo checkUnderscore(@NotNull PsiIdentifier identifier, @NotNull PsiVariable variable) {
+    if ("_".equals(variable.getName()) && PsiUtil.isLanguageLevel8OrHigher(variable)) {
+      if (variable instanceof PsiParameter && ((PsiParameter)variable).getDeclarationScope() instanceof PsiLambdaExpression) {
+        String message = JavaErrorMessages.message("underscore.lambda.identifier");
+        return HighlightInfo.newHighlightInfo(HighlightInfoType.ERROR).range(identifier).descriptionAndTooltip(message).create();
+      }
+      else {
+        String message = JavaErrorMessages.message("underscore.identifier");
+        return HighlightInfo.newHighlightInfo(HighlightInfoType.WARNING).range(identifier).descriptionAndTooltip(message).create();
+      }
+    }
+
+    return null;
+  }
+
   @NotNull
   public static String formatClass(@NotNull PsiClass aClass) {
     return formatClass(aClass, true);

java/java-impl/src/com/intellij/codeInsight/daemon/impl/analysis/HighlightVisitorImpl.java

@@ -515,18 +515,19 @@ public class HighlightVisitorImpl extends JavaElementVisitor implements Highligh
 
   @Override
   public void visitForeachStatement(final PsiForeachStatement statement) {
-    myHolder.add(HighlightUtil.checkForEachFeature(statement, myLanguageLevel,myFile));
+    myHolder.add(HighlightUtil.checkForEachFeature(statement, myLanguageLevel, myFile));
   }
 
   @Override
   public void visitImportStaticStatement(final PsiImportStaticStatement statement) {
-    myHolder.add(HighlightUtil.checkStaticImportFeature(statement, myLanguageLevel,myFile));
+    myHolder.add(HighlightUtil.checkStaticImportFeature(statement, myLanguageLevel, myFile));
   }
 
   @Override
   public void visitIdentifier(final PsiIdentifier identifier) {
+    TextAttributesScheme colorsScheme = myHolder.getColorsScheme();
+
     PsiElement parent = identifier.getParent();
-    final TextAttributesScheme colorsScheme = myHolder.getColorsScheme();
     if (parent instanceof PsiVariable) {
       PsiVariable variable = (PsiVariable)parent;
       myHolder.add(HighlightUtil.checkVariableAlreadyDefined(variable));
@@ -535,6 +536,7 @@ public class HighlightVisitorImpl extends JavaElementVisitor implements Highligh
         final PsiElement child = variable.getLastChild();
         if (child instanceof PsiErrorElement && child.getPrevSibling() == identifier) return;
       }
+
       boolean isMethodParameter = variable instanceof PsiParameter && ((PsiParameter)variable).getDeclarationScope() instanceof PsiMethod;
       if (!isMethodParameter) { // method params are highlighted in visitMethod since we should make sure the method body was visited before
         if (HighlightControlFlowUtil.isReassigned(variable, myFinalVarProblems)) {
@@ -547,6 +549,8 @@ public class HighlightVisitorImpl extends JavaElementVisitor implements Highligh
       else {
         myReassignedParameters.put((PsiParameter)variable, 1); // mark param as present in current file
       }
+
+      myHolder.add(HighlightUtil.checkUnderscore(identifier, variable));
     }
     else if (parent instanceof PsiClass) {
       PsiClass aClass = (PsiClass)parent;
@@ -569,6 +573,7 @@ public class HighlightVisitorImpl extends JavaElementVisitor implements Highligh
     else {
       visitParentReference(parent);
     }
+
     super.visitIdentifier(identifier);
   }
 

java/java-psi-impl/src/messages/JavaErrorMessages.properties

@@ -363,6 +363,9 @@ override.not.allowed.in.interfaces=@Override is not allowed when implementing in
 wildcard.not.expected=Unexpected wildcard
 bound.not.expected=Unexpected bound
 
+underscore.identifier=Use of '_' as an identifier might not be supported in releases after Java 8
+underscore.lambda.identifier=Use of '_' as a lambda parameter name is not allowed
+
 feature.generics=Generics
 feature.annotations=Annotations
 feature.static.imports=Static imports

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/lambda/highlighting/Underscores.java

@@ -0,0 +1,16 @@
+class C {
+  void test() {
+    {
+      I <warning descr="Use of '_' as an identifier might not be supported in releases after Java 8">_</warning> = new I() { public void f(int i) { } };
+      accept(_);
+    }
+
+    {
+      accept(<error descr="Use of '_' as a lambda parameter name is not allowed">_</error> -> System.out.println(_));
+      accept((int <error descr="Use of '_' as a lambda parameter name is not allowed">_</error>) -> System.out.println(_));
+    }
+  }
+
+  interface I { void f(int i); }
+  void accept(I i) { i.f(42); }
+}

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

@@ -86,10 +86,11 @@ public class LambdaHighlightingTest extends LightDaemonAnalyzerTestCase {
   public void testVoidCompatibility() { doTest(); }
   public void testConditionalInferenceFromOppositePart() { doTest(); }
   public void testDeclaredTypeParameterBoundsAndUnboundedWildcard() { doTest(); }
-  public void testConflictResolution() throws Exception {doTest();}
-  public void testIDEA108195() throws Exception {doTest();}
-  public void testDiamondInference() throws Exception { doTest();}
-  public void testFunctionalInterfaceCheck() throws Exception { doTest();}
+  public void testConflictResolution() { doTest(); }
+  public void testIDEA108195() { doTest(); }
+  public void testDiamondInference() { doTest();}
+  public void testFunctionalInterfaceCheck() { doTest();}
+  public void testUnderscores() { doTest(true);}
 
   private void doTest() {
     doTest(false);