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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/99186] std::tuple compilation error when elements are
 specializations of template class declared with template < auto E > syntax
 with E being a enumerator of a enum
Date: Wed, 08 Dec 2021 15:49:26 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99186

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|diagnostic                  |

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Reduced to remove the library dependency:

template<int N, typename T, typename... U>
struct tuple_impl : tuple_impl<N + 1, U...>
{ };

template<int N, typename T>
struct tuple_impl<N, T>
{ };

template<typename T, typename U> struct tuple : tuple_impl<0, T, U> { };

template<typename T, int N, typename... U>
void
get(const tuple_impl<N, T, U...>&)
{ }

enum class E1 {a};
enum class E2 {b,c};

template < auto >
struct S
{
    int i;
};

int main()
{
   tuple<S<E1::a>,S<E2::b>> x;
   get<S<E2::b>>(x); // does not compile
}

Clang and EDG compile this, G++ has a bogus ambiguity:

ambig.C: In function =E2=80=98int main()=E2=80=99:
ambig.C:28:18: error: invalid initialization of reference of type =E2=80=98=
const
tuple_impl<0, S<E2::b>, S<E2::b> >&=E2=80=99 from expression of type =E2=80=
=98tuple<S<E1::a>,
S<E2::b> >=E2=80=99
   28 |    get<S<E2::b>>(x); // does not compile
      |                  ^
ambig.C:13:5: note: in passing argument 1 of =E2=80=98void get(const tuple_=
impl<N, T, U
...>&) [with T =3D S<E2::b>; int N =3D 0; U =3D {S<E2::b>}]=E2=80=99
   13 | get(const tuple_impl<N, T, U...>&)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The bug is that S<E1::a> and S<E2::b> are distinct types but the compiler g=
ets
them confuses.

We have several bugs about template<auto> types that get confused.=