8203338: Unboxing in return from lambda miscompiled to throw ClassCastException

Reviewed-by: mcimadamore, vromero

src/jdk.compiler/share/classes/com/sun/tools/javac/comp/LambdaToMethod.java

@@ -655,9 +655,8 @@ public class LambdaToMethod extends TreeTranslator {
                 return make.Block(0, stats.toList());
             } else {
                 //non-void to non-void conversion:
-                // return (TYPE)BODY;
-                JCExpression retExpr = transTypes.coerce(attrEnv, expr, restype);
-                return make.at(retExpr).Block(0, List.of(make.Return(retExpr)));
+                // return BODY;
+                return make.at(expr).Block(0, List.of(make.Return(expr)));
             }
         } finally {
             make.at(prevPos);
@@ -692,11 +691,6 @@ public class LambdaToMethod extends TreeTranslator {
                     VarSymbol loc = makeSyntheticVar(0, names.fromString("$loc"), tree.expr.type, lambdaMethodDecl.sym);
                     JCVariableDecl varDef = make.VarDef(loc, tree.expr);
                     result = make.Block(0, List.of(varDef, make.Return(null)));
-                } else if (!isTarget_void || !isLambda_void) {
-                    //non-void to non-void conversion:
-                    // return (TYPE)RET-EXPR;
-                    tree.expr = transTypes.coerce(attrEnv, tree.expr, restype);
-                    result = tree;
                 } else {
                     result = tree;
                 }

src/jdk.compiler/share/classes/com/sun/tools/javac/comp/TransTypes.java

@@ -476,11 +476,11 @@ public class TransTypes extends TreeTranslator {
         result = tree;
     }
 
-    JCTree currentMethod = null;
+    Type returnType = null;
     public void visitMethodDef(JCMethodDecl tree) {
-        JCTree previousMethod = currentMethod;
+        Type prevRetType = returnType;
         try {
-            currentMethod = tree;
+            returnType = erasure(tree.type).getReturnType();
             tree.restype = translate(tree.restype, null);
             tree.typarams = List.nil();
             tree.params = translateVarDefs(tree.params);
@@ -490,7 +490,7 @@ public class TransTypes extends TreeTranslator {
             tree.type = erasure(tree.type);
             result = tree;
         } finally {
-            currentMethod = previousMethod;
+            returnType = prevRetType;
         }
     }
 
@@ -533,11 +533,11 @@ public class TransTypes extends TreeTranslator {
     }
 
     public void visitLambda(JCLambda tree) {
-        JCTree prevMethod = currentMethod;
+        Type prevRetType = returnType;
         try {
-            currentMethod = null;
+            returnType = erasure(tree.getDescriptorType(types)).getReturnType();
             tree.params = translate(tree.params);
-            tree.body = translate(tree.body, tree.body.type==null? null : erasure(tree.body.type));
+            tree.body = translate(tree.body, tree.body.type == null || returnType.hasTag(VOID) ? null : returnType);
             if (!tree.type.isIntersection()) {
                 tree.type = erasure(tree.type);
             } else {
@@ -546,7 +546,7 @@ public class TransTypes extends TreeTranslator {
             result = tree;
         }
         finally {
-            currentMethod = prevMethod;
+            returnType = prevRetType;
         }
     }
 
@@ -601,7 +601,8 @@ public class TransTypes extends TreeTranslator {
     }
 
     public void visitReturn(JCReturn tree) {
-        tree.expr = translate(tree.expr, currentMethod != null ? types.erasure(currentMethod.type).getReturnType() : null);
+        if (!returnType.hasTag(VOID))
+            tree.expr = translate(tree.expr, returnType);
         result = tree;
     }
 

test/langtools/tools/javac/lambda/LambdaReturnUnboxing.java

@@ -0,0 +1,49 @@
+/*
+ * Copyright (c) 2018, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @bug 8203338
+ * @summary Unboxing in return from lambda miscompiled to throw ClassCastException
+ */
+
+import java.util.*;
+
+public class LambdaReturnUnboxing {
+    interface C {
+        Character get(Character c);
+    }
+    static <T> T c(T t, int... i) { return t; }
+
+    static Character d() { return List.of('d').get(0); }
+
+    public static void main(String... args) {
+        List.of('x', 'y').stream().max((a, b) -> List.of(a).get(0));
+        List.of('x', 'y').stream().max((a, b) -> { return List.of(a).get(0); });
+
+        C c = LambdaReturnUnboxing::c;
+        int i = c.get('c');
+
+        i = d();
+    }
+}