[Bug c++/97059] C++20: compound requirement uses inconsistent return type (adds ref)

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97059

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu.org
         Resolution|---                         |INVALID
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #3 from Jason Merrill <jason at gcc dot gnu.org> ---
(In reply to Dimitri Gorokhovik from comment #0)
> template <typename T>
> concept S = requires (T t)
> {
>   { t.n } -> std::same_as <const std::size_t
> #if NOBUG
> &
> #endif
> >;
>   requires std::is_same_v <decltype (t.n), const std::size_t>;
> };
> 
> Compilation succeeds when NOBUG is defined, i.e., the compound requirement
> '{ t.n } ->' produces the return type as 'const std::size_t &' instead of
> 'const std::size_t'. Which is inconsistent elsewhere (see the source code
> above).

For the second 'requires' to be equivalent to the return-type-requirement, you
need another set of parentheses around "t.n".

7.5.7.3: "The immediately-declared constraint (13.2) of the type-constraint for
decltype((E)) shall be satisfied."

It's the extra parentheses that result in the reference type rather than
non-reference.  GCC is correct here.