[Bug c++/96331] Class template argument deduction (CTAD) with Concepts

[rs2740 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96331

TC <rs2740 at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rs2740 at gmail dot com

--- Comment #2 from TC <rs2740 at gmail dot com> ---
(In reply to Jonathan Wakely from comment #1)
> (In reply to Nicole from comment #0)
> > template<fixed_string NAME, typename TYPE>
> 
> This is not valid. fixed_string is a class template, not a type and not a
> concept. So you can't use it as a template parameter.
> 
> I don't think this is a bug.

It is a placeholder for a deduced class type, which is allowed by
[temp.param]/6.3.

The example can be reduced drastically:

template<int N = 0>
struct S { };

template<S>
concept C = true;

static_assert(C<S<0>{}>); // OK

template<S Value>
void f() requires C<Value> { // error
}

<source>:11:19: error: class template argument deduction failed:

   11 | void f() requires C<Value> {

      |                   ^~~~~~~~

<source>:11:19: error: no matching function for call to 'S(S<...auto...>)'

<source>:2:8: note: candidate: 'template<int N> S()-> S<N>'

    2 | struct S {

      |        ^

<source>:2:8: note:   template argument deduction/substitution failed:

<source>:11:19: note:   candidate expects 0 arguments, 1 provided

   11 | void f() requires C<Value> {

      |                   ^~~~~~~~

<source>:2:8: note: candidate: 'template<int N> S(S<N>)-> S<N>'

    2 | struct S {

      |        ^

<source>:2:8: note:   template argument deduction/substitution failed:

<source>:11:19: note:   mismatched types 'S<N>' and 'S<...auto...>'

   11 | void f() requires C<Value> {

      |                   ^~~~~~~~

Compiler returned: 1