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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/110580] [14 Regression] gcc fails to typecheck nix-2.16.1
 source: error: invalid initialization of reference of type
Date: Sun, 09 Jul 2023 18:26:49 +0000
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X-Bugzilla-Type: changed
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 14.0
X-Bugzilla-Keywords: needs-bisection, rejects-valid
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X-Bugzilla-Who: pinskia at gcc dot gnu.org
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110580

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Reduced all the way:
```
template <typename _Tp> struct remove_reference {
  using type =3D __remove_reference(_Tp);
};
template <typename _Tp>
using remove_reference_t =3D typename remove_reference<_Tp>::type;
template <typename _Tp, typename _Up>
inline constexpr bool is_same_v =3D __is_same(_Tp, _Up);

template <typename _Key> class s3 {
public:
  template <typename _Up, typename _Vp =3D remove_reference_t<_Up>>
  static constexpr bool __usable_key =3D is_same_v<const _Vp, const _Key>;
  template <typename _Args> void f(_Args __args, int t) {
    if constexpr (__usable_key<_Args>) {
      const _Key &__k =3D __args;
    }
  }
};
struct s0 {};
struct s1 {};
s1 g();
void createMember(int member, s3<s0> &children) { children.f(g(), member); }

```

Or if using the internal type_traits is bad idea here is better testcase:
```

#include <type_traits>

using namespace std;

template <typename _Key> class s3 {
public:
  template <typename _Up, typename _Vp =3D remove_reference_t<_Up>>
  static constexpr bool __usable_key =3D is_same_v<const _Vp, const _Key>;
  template <typename _Args> void f(_Args __args, int t) {
    if constexpr (__usable_key<_Args>) {
      const _Key &__k =3D __args;
    }
  }
};
struct s0 {};
struct s1 {};
s1 g();
void createMember(int member, s3<s0> &children) { children.f(g(), member); }
```

Here is a testcase using static assert rather than if constexpr:
```
#include <type_traits>

using namespace std;

template <typename _Key> class s3 {
public:
  template <typename _Up, typename _Vp =3D remove_reference_t<_Up>>
  static constexpr bool __usable_key =3D is_same_v<const _Vp, const _Key>;
  template <typename _Args> void f(_Args __args, int t) {
    static_assert(!__usable_key<_Args>);
  }
};
struct s0 {};
struct s1 {};
s1 g();
void createMember(int member, s3<s0> &children) { children.f(g(), member); }
```=