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From: "nickhuang99 at hotmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/104091] New: -std=c++20 causing meaningless error message
 "'auto' not allowed in alias declaration" which should be "missing template
 arguments after ..."
Date: Tue, 18 Jan 2022 11:00:14 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104091

            Bug ID: 104091
           Summary: -std=3Dc++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=3DTemplate;
};


Using -std=3Dc++20 gives meaningless error message of "'auto' not allowed in
alias declaration". While -std=3Dc++17 or before all give correct error mes=
sage:
"missing template arguments after 'Template<...auto...>'". And -std=3Dc++14=
 is
even better with "invalid use of template-name 'Template' without an argume=
nt
list".

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