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From: "nickhuang99 at hotmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/104113] New: DR 625 forbids "auto" being used in template
 argument and parser fails to issue correct error message for it
Date: Wed, 19 Jan 2022 10:46:26 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104113

            Bug ID: 104113
           Summary: DR 625 forbids "auto" being used in template argument
                    and parser fails to issue correct error message for it
           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: ---

DR 625 forbids "auto" to be used in template argument. And developer has
specific error message for this and fails to issue it for the very case:
(https://www.godbolt.org/z/198fn1jr1)

template <class T> struct A {};
template <class T> void f(A<T> x) {}

void g()
{
    f(A<short>());

    A<auto> x =3D A<short>();
}


The error message is very misleading. Note that only the 2nd error message =
is
parser confirmed error message which is considered as conversion issue inst=
ead
of "auto" being forbidden to be in template argument. (The 1st error messag=
e is
parser tentative parsing issued.)

<source>: In function 'void g()':
<source>:14:11: error: template argument 1 is invalid
   14 |     A<auto> x =3D A<short>();
      |           ^
<source>:14:17: error: cannot convert 'A<short int>' to 'int' in initializa=
tion
   14 |     A<auto> x =3D A<short>();
      |                 ^~~~~~~~~~
      |                 |
      |                 A<short int>



And this case can be reduced as=20

template<typename T>
struct A{};
A<auto> x =3D A<short>();

This is related to PR104091, however, it is very different because this one=
 is
not affected by "-std=3Dc++XX"=