Why does this rvalue reference bind to an lvalue?

Question

I do not understand why the following code compiles on GCC 8.0:

decltype(auto) foo(int&& r) {
    return r;
}

In foo, the declaration type of r is int&&, and so the return type of foo is also int&&. But r itself is an lvalue, and an lvalue cannot bind to an rvalue reference.

Am I missing something?

Reproduced with [gcc8](https://wandbox.org/permlink/4CSGrHnZ26nGL8QK), it seems gcc8 deduced the return type to be `int&`. — songyuanyao, Aug 06 '17 at 12:33
@songyuanyao They claim the code compiles, so that would be no repro. — Baum mit Augen, Aug 06 '17 at 12:34
@BaummitAugen The fixed code compiles with [gcc8](https://wandbox.org/permlink/7mUFIa2VVAwgprEM), but not [clang6](https://wandbox.org/permlink/3lpIoyciSWUisYh7). — songyuanyao, Aug 06 '17 at 12:37
@songyuanyao sorry for the confusion; `foo` should not compile for reasons given in the question — curiousguy12, Aug 06 '17 at 12:42
Seems like a GCC bug indeed. Note however that if you were to return a parenthesized id expression (`return (r);`, it would work by design). — StoryTeller - Unslander Monica, Aug 06 '17 at 13:13
@curiousguy12: As an aside, welcome to StackOverflow. Great first question, we need more people like you around. — AndyG, Aug 06 '17 at 18:57

Columbo · Accepted Answer · 2017-08-07T11:47:09.693

According to [dcl.spec.auto]/5, the return type is deduced as if the return statement's operand was the operand of decltype. And [dcl.type.simple]/(4.2) clearly states that, as the operand is not parenthesized, the type of the entity is the type yielded by decltype, that is, int&&. And indeed, r is an lvalue ([expr.prim.id.unqual]).

Fortunately, this has been discovered and filed as bug 64892 two years ago. (I wonder why no one could find the time to fix this?)

Why does this rvalue reference bind to an lvalue?

1 Answers1