[Bug c++/104091] New: -std=c++20 causing meaningless error message "'auto' not allowed in alias declaration" which should be "missing template arguments after ..."

[Bug c++/104091] New: -std=c++20 causing meaningless error message "'auto' not allowed in alias declaration" which should be "missing template arguments after ..."

[nickhuang99 at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104091

            Bug ID: 104091
           Summary: -std=c++20 causing meaningless error message "'auto'
                    not allowed in alias declaration" which should be
                    "missing template arguments after ..."
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nickhuang99 at hotmail dot com
  Target Milestone: ---

considering following snippet of code:

template<typename T, template<typename...>typename Template>
struct Specialization{};
template<template<typename...>typename Template, typename...Args>
struct Specialization<Template<Args...>, Template>{
    using type=Template;
};


Using -std=c++20 gives meaningless error message of "'auto' not allowed in
alias declaration". While -std=c++17 or before all give correct error message:
"missing template arguments after 'Template<...auto...>'". And -std=c++14 is
even better with "invalid use of template-name 'Template' without an argument
list".

See https://godbolt.org/z/Wb9nKzcPT

[Bug c++/104091] -std=c++20 causing meaningless error message "'auto' not allowed in alias declaration" which should be "missing template arguments after ..."

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104091

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2022-01-18
     Ever confirmed|0                           |1
           Keywords|                            |diagnostic
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed reduced to just:
template<typename> class Tt;
using type=Tt;

[Bug c++/104091] -std=c++20 causing meaningless error message "'auto' not allowed in alias declaration" which should be "missing template arguments after ..."

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104091

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2022-01-18 00:00:00         |2022-11-7

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I've just hit this myself and was quite confused by the diagnostic.

A realistic but still close to minimal reproducer is:

namespace x
{
  template<class T> struct S { };
}

struct A
{
  using S = ::x::S;
};


Unsurprisingly, you get the same error with -std=c++17 -fconcepts, because the
compiler seems to be treating the template-id as a placeholder-type-specifier,
maybe even as a type-constraint. Neither makes sense as the defining-type-id of
an alias-declaration.


With -std=c++14 you get a correct "invalid use of template-name" error, but
then an incorrect note about CTAD:

using.cc:8:13: error: invalid use of template-name 'x::S' without an argument
list
    8 |   using S = ::x::S;
      |             ^~
using.cc:8:13: note: class template argument deduction is only available with
'-std=c++17' or '-std=gnu++17'
using.cc:3:28: note: 'template<class T> struct x::S' declared here
    3 |   template<class T> struct S { };
      |                            ^

Although it's true that CTAD isn't available in C++14, it's not relevant here
because it couldn't be used here anyway (as evidenced by the fact it still
doesn't compie in C++17).


I would expect the same error for C++14 and C++17 (and ideally C++20 too) since
the code is invalid in exactly the same way in all cases. C++17 gives a similar
error to C++14 but with slightly different wording for some reason:

using.cc:8:13: error: missing template arguments after 'x::S'
    8 |   using S = ::x::S;
      |             ^~
      |               <>
using.cc:3:28: note: 'template<class T> struct x::S' declared here
    3 |   template<class T> struct S { };
      |                            ^


So we should:

1) fix the bogus "auto not allowed" when concepts are enabled;
2) harmonize the C++14 and C++17 wording about missing template args;
3) remove the bogus note about CTAD.

[Bug c++/104091] -std=c++20 causing meaningless error message "'auto' not allowed in alias declaration" which should be "missing template arguments after ..."

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104091

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |Theodore.Papadopoulo@inria.
                   |                            |fr

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
*** Bug 101221 has been marked as a duplicate of this bug. ***

[Bug c++/104091] -std=c++20 causing meaningless error message "'auto' not allowed in alias declaration" which should be "missing template arguments after ..."

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104091

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
A slight variation from PR 101221:

template<class T> struct S;
template<class T> using SS = S;


For comparison, Clang and EDG give exactly the same diagnostic for all examples
above irrespective of C++14/17/20 mode.