unused return value inspection: method reference awareness (IDEA-134991)

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

@@ -211,6 +211,14 @@ public class RefJavaUtilImpl extends RefJavaUtil{
     PsiMethod psiMethod = (PsiMethod)psiResolved;
     RefMethodImpl refMethod = (RefMethodImpl)refResolved;
 
+    if (refExpression instanceof PsiMethodReferenceExpression) {
+      PsiType returnType = psiMethod.getReturnType();
+      if (!psiMethod.isConstructor() && returnType != PsiType.VOID) {
+        refMethod.setReturnValueUsed(true);
+        addTypeReference(psiFrom, returnType, refFrom.getRefManager());
+      }
+      return;
+    }
     PsiMethodCallExpression call = PsiTreeUtil.getParentOfType(
       refExpression,
       PsiMethodCallExpression.class

java/java-tests/testData/inspection/unusedReturnValue/methodReference/expected.xml

@@ -0,0 +1,2 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<problems/>

java/java-tests/testData/inspection/unusedReturnValue/methodReference/src/B.java

@@ -0,0 +1,13 @@
+import java.util.function.Consumer;
+import java.util.function.DoubleSupplier;
+
+class B {
+
+  public void test(Consumer<DoubleSupplier> consumer) {
+    consumer.accept(this::method);
+  }
+
+  private double method() {
+    return 1;
+  }
+}

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

@@ -12,6 +12,10 @@ package com.intellij.codeInspection;
 
 import com.intellij.JavaTestUtil;
 import com.intellij.codeInspection.unusedReturnValue.UnusedReturnValue;
+import com.intellij.openapi.projectRoots.Sdk;
+import com.intellij.openapi.roots.LanguageLevelProjectExtension;
+import com.intellij.pom.java.LanguageLevel;
+import com.intellij.testFramework.IdeaTestUtil;
 import com.intellij.testFramework.InspectionTestCase;
 
 public class UnusedReturnValueTest extends InspectionTestCase {
@@ -39,7 +43,17 @@ public class UnusedReturnValueTest extends InspectionTestCase {
     doTest();
   }
 
-  
+  public void testMethodReference() throws Exception {
+    doTest();
+  }
+
+  @Override
+  protected Sdk getTestProjectSdk() {
+    Sdk sdk = IdeaTestUtil.getMockJdk18();
+    LanguageLevelProjectExtension.getInstance(getProject()).setLanguageLevel(LanguageLevel.JDK_1_8);
+    return sdk;
+  }
+
   public void testSimpleSetter() throws Exception {
     try {
       myTool.IGNORE_BUILDER_PATTERN = true;