[Bug c++/95015] New: Partial specializations of class templates with class NTTP fails

[Bug c++/95015] New: Partial specializations of class templates with class NTTP fails

[mateusz.pusz at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95015

            Bug ID: 95015
           Summary: Partial specializations of class templates with class
                    NTTP fails
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mateusz.pusz at gmail dot com
  Target Milestone: ---

The following code does not compile:

```
template<typename T>
struct X {
  T a;
  int b;
};
template<typename T>
X(T, int) -> X<T>;  // <- does not work
// X(T) -> X<T>;  // < works

template<X V>
struct Y {};

template<typename T>
inline constexpr bool is_Y = false;
template<X NTTP>
inline constexpr bool is_Y<Y<NTTP>> = true;

X x(3, 3);
Y<X(3, 3)> y;
using my_y = Y<X(3, 3)>;
static_assert(is_Y<my_y>);
```

The error is:

```
<source>:16:30: error: class template argument deduction failed:
   16 | inline constexpr bool is_Y<Y<NTTP>> = true;
      |                              ^~~~
<source>:16:30: error: no matching function for call to 'X(X<...auto...>)'
<source>:2:8: note: candidate: 'template<class T> X()-> X<T>'
    2 | struct X {
      |        ^
<source>:2:8: note:   template argument deduction/substitution failed:
<source>:16:30: note:   candidate expects 0 arguments, 1 provided
   16 | inline constexpr bool is_Y<Y<NTTP>> = true;
      |                              ^~~~
<source>:2:8: note: candidate: 'template<class T> X(X<T>)-> X<T>'
    2 | struct X {
      |        ^
<source>:2:8: note:   template argument deduction/substitution failed:
<source>:16:30: note:   mismatched types 'X<T>' and 'X<...auto...>'
   16 | inline constexpr bool is_Y<Y<NTTP>> = true;
      |                              ^~~~
<source>:7:1: note: candidate: 'template<class T> X(T, int)-> X<T>'
    7 | X(T, int) -> X<T>;  // <- does not work
      | ^
<source>:7:1: note:   template argument deduction/substitution failed:
<source>:16:30: note:   candidate expects 2 arguments, 1 provided
   16 | inline constexpr bool is_Y<Y<NTTP>> = true;
      |                              ^~~~
<source>:16:23: error: template argument 1 is invalid
   16 | inline constexpr bool is_Y<Y<NTTP>> = true;
      |                       ^~~~~~~~~~~~~
<source>:16:23: error: template-id 'is_Y<<expression error> >' for 'is_Y' does
not match any template declaration
<source>:14:23: note: candidate is: 'template<class T> constexpr const bool
is_Y<T>'
   14 | inline constexpr bool is_Y = false;
      |                       ^~~~
<source>:21:15: error: static assertion failed
   21 | static_assert(is_Y<my_y>);
      |               ^~~~~~~~~~
Compiler returned: 1
```

https://godbolt.org/z/597fXY

[Bug c++/95015] Partial specializations of class templates with class NTTP fails

[johelegp at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95015

Johel Ernesto Guerrero Peña <johelegp at gmail dot com> changed:

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

--- Comment #1 from Johel Ernesto Guerrero Peña <johelegp at gmail dot com> ---
Seems fixed in trunk, with and without the deduction guide.

[Bug c++/95015] Partial specializations of class templates with class NTTP fails

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95015

Patrick Palka <ppalka at gcc dot gnu.org> changed:

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

--- Comment #2 from Patrick Palka <ppalka at gcc dot gnu.org> ---
(In reply to Johel Ernesto Guerrero Peña from comment #1)
> Seems fixed in trunk, with and without the deduction guide.

Ever since r11-5752, so dup of PR93083 apparently.

*** This bug has been marked as a duplicate of bug 93083 ***