[cfe-dev] Structured bindings seem to allow non-const references to a bitfield

[James Dennett via cfe-dev]

On Wed, Mar 22, 2017 at 12:01 PM, Keane, Erich via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> I wasn't sure whether to bring this up here, or to bring it up on the
> reflector, but I'm not sure if this is a compiler issue or a standards
> issue.  I read through the structured binding wording, and don't see
> anything one way or the other, and GCC7 and Clang trunk both allow the
> following:
>
> struct F {
>   unsigned a : 1;
>   unsigned b : 2;
>   unsigned c : 8;
> };
>
> int main() {
>   F f;
>   f.a = 0;
>   f.b = 1;
>   f.c = 4;
>
>   auto &b = f.b; // correctly errors on non-const ref to a bitfield
>
>   auto& [d,e,g] = f; // all are references to a bitfield
>

The only reference here is an unnamed reference to `f`.  None of `d`, `e`,
`g` is a reference.  `d` is another name for `f.a`, and similarly for `e`
and `g`.


>   d = 3; // Works, but warns about truncation
>   c = 3; // Works, no diagnostic
> }
>
> Is this intended?  Is this justified by the standard?
>

The behavior you describe appears correct (and unsurprising to me), apart
from the assumption that these are references; they're "structured
bindings", not references.

-- James
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