unused declaration: add refs on functional interface from functional expressions (IDEA-132138)

java/java-analysis-impl/src/com/intellij/codeInspection/reference/RefJavaUtilImpl.java

@@ -92,6 +92,29 @@ public class RefJavaUtilImpl extends RefJavaUtil{
             }
           }
 
+          @Override
+          public void visitLambdaExpression(PsiLambdaExpression expression) {
+            super.visitLambdaExpression(expression);
+            processFunctionalExpression(expression);
+          }
+
+          @Override
+          public void visitMethodReferenceExpression(PsiMethodReferenceExpression expression) {
+            super.visitMethodReferenceExpression(expression);
+            processFunctionalExpression(expression);
+          }
+
+          private void processFunctionalExpression(PsiFunctionalExpression expression) {
+            final PsiClass aClass = PsiUtil.resolveClassInType(expression.getFunctionalInterfaceType());
+            if (aClass != null) {
+              refFrom.addReference(refFrom.getRefManager().getReference(aClass), aClass, psiFrom, false, true, null);
+              final PsiMethod interfaceMethod = LambdaUtil.getFunctionalInterfaceMethod(aClass);
+              if (interfaceMethod != null) {
+                refFrom.addReference(refFrom.getRefManager().getReference(interfaceMethod), interfaceMethod, psiFrom, false, true, null);
+              }
+            }
+          }
+
           @Nullable
           private RefMethod processNewLikeConstruct(final PsiMethod psiConstructor, final PsiExpressionList argumentList) {
             if (psiConstructor != null) {

java/java-tests/testData/inspection/deadCode/functionalExpressions/expected.xml

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<problems/>
+
+

java/java-tests/testData/inspection/deadCode/functionalExpressions/src/Test.java

@@ -0,0 +1,10 @@
+public class Test {
+  public static void main(String[] args) {
+    final Memento m = () -> isPlaying;
+    System.out.println(m);
+  }
+
+  public static interface Memento {
+    boolean isPlaying();
+  }
+}

java/java-tests/testSrc/com/intellij/codeInspection/UnusedDeclarationTest.java

@@ -190,4 +190,9 @@ public class UnusedDeclarationTest extends InspectionTestCase {
   public void testClassLiteralRef() {
     doTest();
   }
+
+  public void testFunctionalExpressions() {
+    LanguageLevelProjectExtension.getInstance(getJavaFacade().getProject()).setLanguageLevel(LanguageLevel.JDK_1_8);
+    doTest();
+  }
 }