expand one line lambda: fix value compatibility check

plugins/IntentionPowerPak/src/com/siyeh/ipp/types/ExpandOneLineLambda2CodeBlockIntention.java

@@ -44,7 +44,7 @@ public class ExpandOneLineLambda2CodeBlockIntention extends Intention {
     final PsiElement body = lambdaExpression.getBody();
     LOG.assertTrue(body instanceof PsiExpression);
     String blockText = "{";
-    blockText += ((PsiExpression)body).getType() == PsiType.VOID ? "" : "return ";
+    blockText += PsiType.VOID.equals(LambdaUtil.getFunctionalInterfaceReturnType(lambdaExpression)) ? "" : "return ";
     blockText +=  body.getText() + ";}";
 
     final String resultedLambdaText = lambdaExpression.getParameterList().getText() + "->" + blockText;

plugins/IntentionPowerPak/test/com/siyeh/ipp/types/expr2block/VoidCompatibleInExpr.java

@@ -0,0 +1,7 @@
+import java.util.List;
+
+class X {
+  void foo(List<String> list){
+    Runnable c = () <caret>-> list.add("");
+  }
+}

plugins/IntentionPowerPak/test/com/siyeh/ipp/types/expr2block/VoidCompatibleInExpr_after.java

@@ -0,0 +1,9 @@
+import java.util.List;
+
+class X {
+  void foo(List<String> list){
+    Runnable c = () -> {
+        list.add("");
+    };
+  }
+}

plugins/IntentionPowerPak/testSrc/com/siyeh/ipp/types/ExpandOneLineLambda2CodeBlockIntentionTest.java

@@ -22,6 +22,11 @@ public class ExpandOneLineLambda2CodeBlockIntentionTest extends IPPTestCase {
   public void testSimple() {
     doTest();
   }
+
+  public void testVoidCompatibleInExpr() {
+    doTest();
+  }
+
   @Override
   protected String getIntentionName() {
     return "Expand lambda expression body to {...}";