RFR: JDK-8203338: Unboxing in return from lambda miscompiled to throw ClassCastException

[Vicente Romero]

Hi,

Not sure about this one. I think that the type of the return expression 
inside a lambda should be erased to the erasure of the return type of 
the lambda type. The `currentMethod` field at TransTypes is just a 
carrier to actually access the erasure of the return type of the current 
method, and this access happens only inside visitReturn. So I wonder if 
we actually should have a field in TransTypes that could hold that 
erasure directly. At TransTypes.visitMethodDef we could have:

theField = types.erasure(tree.type.getReturnType());

and at TransTypes.visitLambda it could be:

theField = 
types.erasure(tree.getDescriptorType(types).asMethodType().restype);

of course with a better name, and then at TransTypes.visitReturn, it 
could use the field to erase the returned expression.

Thanks,
Vicente

On 06/16/2018 06:20 PM, B. Blaser wrote:
> Hi,
>
> As noted in the JBS issue, the following example fails at run-time with a CCE:
>
>      List.of('x', 'y').stream().max((a, b) -> { return List.of(a).get(0); });
>
> The interesting point here is that removing 'return' makes it run successfully:
>
>      List.of('x', 'y').stream().max((a, b) -> List.of(a).get(0));
>
> The problem seems to be in 'TransType.visitReturn()'. The lambda
> method doesn't yet exist ('currentMethod == null', see
> 'TransType.visitLambda()') causing the return expression type to be
> set to the erasure of 'List.get()' (= Object) which further causes the
> type conversion problem (see
> 'LambdaToMethod.makeLambdaStatementBody()').
>
> The suggested fix (here under) is to keep the original erased
> expression type (= Character) if 'currentMethod == null'.
>
> Tier1 is OK.
>
> Any feedback is welcome,
> Bernard
>
> diff -r ed8de3d0cd28
> src/jdk.compiler/share/classes/com/sun/tools/javac/comp/TransTypes.java
> --- a/src/jdk.compiler/share/classes/com/sun/tools/javac/comp/TransTypes.java
>     Sat Jun 16 10:10:54 2018 +0100
> +++ b/src/jdk.compiler/share/classes/com/sun/tools/javac/comp/TransTypes.java
>     Sat Jun 16 21:16:33 2018 +0200
> @@ -601,7 +601,8 @@
>       }
>
>       public void visitReturn(JCReturn tree) {
> -        tree.expr = translate(tree.expr, currentMethod != null ?
> types.erasure(currentMethod.type).getReturnType() : null);
> +        tree.expr = translate(tree.expr, currentMethod != null ?
> types.erasure(currentMethod.type).getReturnType() :
> +                tree.expr != null ? types.erasure(tree.expr.type) : null);
>           result = tree;
>       }