[Bug c++/102493] New: non-type template specialization for member pointer to field and function reports leads to unexpected conflict

[Bug c++/102493] New: non-type template specialization for member pointer to field and function reports leads to unexpected conflict

[marekr22 at wp dot pl]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102493

            Bug ID: 102493
           Summary: non-type template specialization for member pointer to
                    field and function reports leads to unexpected
                    conflict
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: marekr22 at wp dot pl
  Target Milestone: ---

Here is minimum complete verifiable example (C++17):
```cpp
#include <iostream>
#include <string>
#include <type_traits>

template <auto Fp>
struct field_type;

template<typename R, typename T, R (T::*FP)>
struct field_type<FP>
{
    using type = R;
};

template<typename R, typename T, typename...Args, R (T::*FP)(Args...)>
struct field_type<FP>
{
    using type = R;
}; 

template<auto FP>
using field_type_t = typename field_type<FP>::type;


class Foo {
public:
    int x = 0;
    double y = 0;
    std::string s;
    const int cx = 0;

    Foo() = default;

    void bar() {
        std::cout << "bar\n";
    }

    int par(int z) {
        std::cout << "bar\n";
        return z;
    }
};

template<auto F, typename T>
constexpr bool test = std::is_same_v<field_type_t<F>, T>;

static_assert(test<&Foo::x,    int>,         "");
static_assert(test<&Foo::cx,   const int>,   "");
static_assert(test<&Foo::s,    std::string>, "");
static_assert(test<&Foo::y,    double>,      "");
#ifndef HIDE_PROBLEM_ON_GCC_11
static_assert(test<&Foo::bar,  void>,        "");
static_assert(test<&Foo::par,  int>,         "");
#endif
```

This compile on all compilers https://godbolt.org/z/31svobz3z
except for gcc 11.1 and 11.2 (gcc 10.3 works).

Reported error is:
```
<source>: In substitution of 'template<auto FP> using field_type_t = typename
field_type::type [with auto FP = &Foo::bar]':
<source>:44:28:   required from 'constexpr const bool test<&Foo::bar, void>'
<source>:51:15:   required from here
<source>:21:7: error: ambiguous template instantiation for 'struct
field_type<&Foo::bar>'
   21 | using field_type_t = typename field_type<FP>::type;
      |       ^~~~~~~~~~~~
<source>:9:8: note: candidates are: 'template<class R, class T, R T::* FP>
struct field_type<FP> [with R = void(); T = Foo; R T::* FP = &Foo::bar]'
    9 | struct field_type<FP>
      |        ^~~~~~~~~~~~~~
<source>:15:8: note:                 'template<class R, class T, class ...
Args, R (T::* FP)(Args ...)> struct field_type<FP> [with R = void; T = Foo;
Args = {}; R (T::* FP)(Args ...) = &Foo::bar]'
   15 | struct field_type<FP>
      |        ^~~~~~~~~~~~~~
<source>:21:7: error: invalid use of incomplete type 'struct
field_type<&Foo::bar>'
   21 | using field_type_t = typename field_type<FP>::type;
      |       ^~~~~~~~~~~~
<source>:6:8: note: declaration of 'struct field_type<&Foo::bar>'
    6 | struct field_type;
      |        ^~~~~~~~~~
```

[Bug c++/102493] non-type template specialization for member pointer to field and function reports leads to unexpected conflict

[jehelset at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102493

--- Comment #1 from John Eivind Helset <jehelset at gmail dot com> ---
ran into this today as well with:
```cpp
struct X{
  void f(char){}
};

template<auto X>
struct Y{};
template<typename T,typename U,T U::* ptr>
struct Y<ptr>{};
template<typename T,typename U,typename... A,T (U::*ptr)(A...)>
struct Y<ptr>{};

Y<&X::f> y;
```
using gcc 13.1.1, clang trunk accepted same code.