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From: "ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/104255] parsing trailing return type fails with parameter
 pack expansion when two parameter packs at present
Date: Fri, 28 Jan 2022 15:11:32 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104255

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ppalka at gcc dot gnu.org

--- Comment #2 from Patrick Palka <ppalka at gcc dot gnu.org> ---
The error message is obscure, but it seems what GCC has issue with here is =
the
use of the function parameter seq2 in the trailing return type occurring
outside of an unevaluated context.

I'm not totally sure if the testcase is valid
(https://eel.is/c++draft/basic.scope.param#note-1 might suggest it's not?),=
 but
one workaround is to replace the use of seq2 in the trailing return type wi=
th
decltype(seq2){} (which works because index_sequence is an empty type).

Here's a minimal testcase demonstrating the issue

  struct empty { };
  constexpr int f(empty) { return 0; }
  template<int> struct A { };
  auto g(empty e) -> A<f(e)>;

which is rejected with

  error: use of parameter outside function body before =E2=80=98)=E2=80=99 =
token

due to 'e' being used outside of an unevaluated context within the signatur=
e of
the function.=