Why is there an error "no matching function for call to 'A(A<...auto...>)'"?

Question

In C++20, we have better NTTPs, which allows literal class types:

template <typename T>
struct A{};

template <A> // note: it is a placeholder for NTTP A<T>
struct B{};

But when I try to specialize a template for B:

template <typename>
struct is_B{};

template <A x>
struct is_B<B<x>>{}; // error!!!

The compiler (GCC 10 or GCC 11(trunk)) dumps a bunch of errors:

prog.cc:11:15: error: class template argument deduction failed:
   11 | struct is_B<B<x>>{};
      |               ^
prog.cc:11:15: error: no matching function for call to 'A(A<...auto...>)'
prog.cc:2:8: note: candidate: 'template<class T> A()-> A<T>'
    2 | struct A{};
      |        ^
prog.cc:2:8: note:   template argument deduction/substitution failed:
prog.cc:11:15: note:   candidate expects 0 arguments, 1 provided
   11 | struct is_B<B<x>>{};
      |               ^
prog.cc:2:8: note: candidate: 'template<class T> A(A<T>)-> A<T>'
    2 | struct A{};
      |        ^
prog.cc:2:8: note:   template argument deduction/substitution failed:
prog.cc:11:15: note:   mismatched types 'A<T>' and 'A<...auto...>'
   11 | struct is_B<B<x>>{};
      |               ^
prog.cc:11:16: error: template argument 1 is invalid
   11 | struct is_B<B<x>>{};
      |                ^~

I found a solution, which is giving an explicit argument to A:

template <typename T, A<T> x>
struct is_B<B<x>>{};

But it is redundant, and cannot solve the same error when A used in using:

template <A x>
using B_t = B<x>; // same error!!!

So is there another solution?

Answer 1

It seems that GCC doesn't like placeholders for deduced class types as NTTP, which should be accepted as per [temp.param]/6 (emphasis mine):

A non-type template-parameter shall have one of the following (possibly cv-qualified) types:

• a structural type (see below),
• a type that contains a placeholder type ([dcl.spec.auto]), or
• a placeholder for a deduced class type ([dcl.type.class.deduct]).

There's already a related bug report (PR96331, see the example provided by @TC).