[Bug c++/112775] New: Class template partial specialization with decltype(n) is wrongly rejected

[Bug c++/112775] New: Class template partial specialization with decltype(n) is wrongly rejected

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112775

            Bug ID: 112775
           Summary: Class template partial specialization with decltype(n)
                    is wrongly rejected
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fchelnokov at gmail dot com
  Target Milestone: ---

In this program


template <class T, T, T>
class A;

template <auto n>
class A<decltype(n), n, n> {};


class template A has one type parameter T, and two arbitrary non-type
parameters (of type T). And also a specialization of A is provided, having
equal non-type template parameters.

The program is accepted by Clang and MSVC, but not by GCC:

error: partial specialization 'class A<decltype (n), n, n>' is not more
specialized than
    5 | class A<decltype(n), n, n> {};
      |       ^~~~~~~~~~~~~~~~~~~~
note: primary template 'template<class T, T <anonymous>, T <anonymous> > class
A'

Online demo: https://godbolt.org/z/qjYMn8bd6

[Bug c++/112775] Class template partial specialization with decltype(n) is wrongly rejected

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112775

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2023-12-02
             Status|UNCONFIRMED                 |NEW
           Keywords|                            |diagnostic

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed. What is interesting is if we don't use -pedantic-errors, the
following is accepted:
```
template <class T, T a, T b>
class A;
template <auto n>
class A<decltype(n), n, n> {};
A<int, 1, 1> b;
```

And even more if we have 2 different values for the 2/3rd arguments, it is
rejected as an incomplete class.

[Bug c++/112775] Class template partial specialization with decltype(n) is wrongly rejected

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112775

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note the workaround is to do just this:
```
template <class T, T n>
class A<T, n, n> {};
```