[Bug c++/109291] New: type alias template rejects pack

[Bug c++/109291] New: type alias template rejects pack

[ncm at cantrip dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109291

            Bug ID: 109291
           Summary: type alias template rejects pack
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ncm at cantrip dot org
  Target Milestone: ---

template <typename T>
struct type_identity { using type = T; };

template <typename T>
using type_identity_t = typename type_identity<T>::type;

template <typename ...Ts>
struct S1 { using alias1 = typename type_identity<Ts...>::type; };

template <typename ...Ts>
struct S2 { using alias2 = typename type_identity_t<Ts...>; };

int main() {
  S1<int>::alias1 a; // OK
  S2<int>::alias2 b; // Fails
}

// Here, alias1 is fine, but alias2, the same type, is not.
// MSVC accepts both declarations. Clang matches Gcc.

// error: pack expansion argument for non-pack parameter ‘T’ of alias template
// error: expected nested-name-specifier

[Bug c++/109291] type alias template rejects pack

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109291

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |DUPLICATE

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Dup of bug 59498.

There is a defect in the C++ standard here ...

*** This bug has been marked as a duplicate of bug 59498 ***

[Bug c++/109291] type alias template rejects pack

[ncm at cantrip dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109291

--- Comment #2 from ncm at cantrip dot org ---
CWG 1430 is still marked Open, and is anyway only superficially
analogous. Here, there is no need for an alias to be encoded
into a type signature.

[Bug c++/109291] type alias template rejects pack

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109291

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to ncm from comment #2)
> CWG 1430 is still marked Open, and is anyway only superficially
> analogous. Here, there is no need for an alias to be encoded
> into a type signature.

Right and yes PR 59498 is still open so ...