suggest SAM with same primitive return type (IDEA-174020)

java/java-impl/src/com/intellij/codeInsight/FunctionalInterfaceSuggester.java

@@ -112,12 +112,12 @@ public class FunctionalInterfaceSuggester {
         }
 
         final PsiType returnType = method.getReturnType();
-        PsiType interfaceMethodReturnType = interfaceMethod.getReturnType();
-        if (returnType != null && !TypeConversionUtil.isAssignable(returnType, substitutor.substitute(interfaceMethodReturnType))) {
+        PsiType interfaceMethodReturnType = substitutor.substitute(interfaceMethod.getReturnType());
+        if (returnType != null && !TypeConversionUtil.isAssignable(returnType, interfaceMethodReturnType)) {
           return null;
         }
-
-        if (PsiType.VOID.equals(returnType) && !PsiType.VOID.equals(interfaceMethodReturnType)) {
+        
+        if (returnType instanceof PsiPrimitiveType && !returnType.equals(interfaceMethodReturnType)) {
           return null;
         }
 

java/java-tests/testSrc/com/intellij/java/refactoring/FunctionalInterfaceSuggesterTest.java

@@ -0,0 +1,46 @@
+/*
+ * Copyright 2000-2017 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.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.intellij.java.refactoring;
+
+import com.intellij.codeInsight.FunctionalInterfaceSuggester;
+import com.intellij.psi.PsiClass;
+import com.intellij.testFramework.LightProjectDescriptor;
+import com.intellij.testFramework.fixtures.LightCodeInsightFixtureTestCase;
+import org.jetbrains.annotations.NotNull;
+import org.junit.Assert;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.stream.Collectors;
+
+public class FunctionalInterfaceSuggesterTest extends LightCodeInsightFixtureTestCase {
+  @NotNull
+  @Override
+  protected LightProjectDescriptor getProjectDescriptor() {
+    return JAVA_8;
+  }
+
+  public void testPrimitiveReturnTypes() throws Exception {
+    PsiClass aClass = myFixture.addClass("class Foo {double foo(double d) {return d;}}");
+    List<String> suggestedTypes = FunctionalInterfaceSuggester.suggestFunctionalInterfaces(aClass.getMethods()[0])
+      .stream()
+      .map(type -> type.getCanonicalText())
+      .collect(Collectors.toList());
+    Assert.assertTrue(suggestedTypes.containsAll(Arrays.asList("java.util.function.IntToDoubleFunction", 
+                                                               "java.util.function.DoubleUnaryOperator")));
+    Assert.assertFalse(suggestedTypes.contains("java.util.function.LongToIntFunction"));
+  }
+}