https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104091
Jonathan Wakely <redi at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Last reconfirmed|2022-01-18 00:00:00 |2022-11-7
--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I've just hit this myself and was quite confused by the diagnostic.
A realistic but still close to minimal reproducer is:
namespace x
{
template<class T> struct S { };
}
struct A
{
using S = ::x::S;
};
Unsurprisingly, you get the same error with -std=c++17 -fconcepts, because the
compiler seems to be treating the template-id as a placeholder-type-specifier,
maybe even as a type-constraint. Neither makes sense as the defining-type-id of
an alias-declaration.
With -std=c++14 you get a correct "invalid use of template-name" error, but
then an incorrect note about CTAD:
using.cc:8:13: error: invalid use of template-name 'x::S' without an argument
list
8 | using S = ::x::S;
| ^~
using.cc:8:13: note: class template argument deduction is only available with
'-std=c++17' or '-std=gnu++17'
using.cc:3:28: note: 'template<class T> struct x::S' declared here
3 | template<class T> struct S { };
| ^
Although it's true that CTAD isn't available in C++14, it's not relevant here
because it couldn't be used here anyway (as evidenced by the fact it still
doesn't compie in C++17).
I would expect the same error for C++14 and C++17 (and ideally C++20 too) since
the code is invalid in exactly the same way in all cases. C++17 gives a similar
error to C++14 but with slightly different wording for some reason:
using.cc:8:13: error: missing template arguments after 'x::S'
8 | using S = ::x::S;
| ^~
| <>
using.cc:3:28: note: 'template<class T> struct x::S' declared here
3 | template<class T> struct S { };
| ^
So we should:
1) fix the bogus "auto not allowed" when concepts are enabled;
2) harmonize the C++14 and C++17 wording about missing template args;
3) remove the bogus note about CTAD.
