[Bug c++/101221] New: Inaccurate error message for wrong template alias

[Bug c++/101221] New: Inaccurate error message for wrong template alias

[Theodore.Papadopoulo at inria dot fr]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101221

            Bug ID: 101221
           Summary: Inaccurate error message for wrong template alias
           Product: gcc
           Version: 10.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: Theodore.Papadopoulo at inria dot fr
  Target Milestone: ---

Created attachment 51062
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51062&action=edit
Code showing the bug

The attached code produces the following message. The reference of the 'auto'
is at best surprising. Of course the code is wrong but the message is not very
helpful for identifying what is wrong.

mururoa-> g++ --std=c++20 test.C
test.C:16:18: error: ‘auto’ not allowed in alias declaration
   16 | using OtherNew = One::AClass;
      |                  ^~~
test.C:23:17: error: ‘auto’ not allowed in alias declaration
   23 |     using New = One::AClass;
      |                 ^~~

clang does a slightly better job and at least at namespace level explains what
is wrong.

mururoa-> clang++ --std=c++20 test.C
test.C:16:23: error: use of class template 'One::AClass' requires template
arguments
using OtherNew = One::AClass;
                      ^
test.C:4:43: note: template is declared here
    template <Type type,typename T> class AClass;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~       ^
test.C:23:17: error: no type named 'AClass' in namespace 'One'; did you mean
'BClass'?
    using New = One::AClass;
                ^~~~~~~~~~~
                BClass
test.C:18:7: note: 'BClass' declared here
class BClass {
      ^
2 errors generated.

[Bug c++/101221] Inaccurate error message for wrong template alias

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101221

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

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

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The code from the attachment is:

namespace One {
    typedef enum { A, B } Type;

    template <Type type,typename T> class AClass;

    template <typename T>
    class AClass<A,T> { };

    template <typename T>
    class AClass<B,T> { };
}

typedef One::Type NewType;

template <NewType type,typename T>
using OtherNew = One::AClass;

class BClass {

    typedef One::Type Type;

    template <Type type,typename T>
    using New = One::AClass;
};


With -std=gnu++17 we print:

101221.C:16:18: error: missing template arguments after 'AClass<...auto...>'
   16 | using OtherNew = One::AClass;
      |                  ^~~
101221.C:4:43: note: 'template<One::Type type, class T> class One::AClass'
declared here
    4 |     template <Type type,typename T> class AClass;
      |                                           ^~~~~~
101221.C:23:17: error: missing template arguments after 'AClass<...auto...>'
   23 |     using New = One::AClass;
      |                 ^~~
101221.C:4:43: note: 'template<One::Type type, class T> class One::AClass'
declared here
    4 |     template <Type type,typename T> class AClass;
      |                                           ^~~~~~


With -std=gnu++20 we print:

101221.C:16:18: error: 'auto' not allowed in alias declaration
   16 | using OtherNew = One::AClass;
      |                  ^~~
101221.C:23:17: error: 'auto' not allowed in alias declaration
   23 |     using New = One::AClass;
      |                 ^~~


Both diagnostics are wrong.

Clang prints the same error for C++17 and C++20.

[Bug c++/101221] Inaccurate error message for wrong template alias

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101221

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

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

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Reduced:

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

template<class T>
using S = x::S;


Which means this is the same as 104091. This is older, but that one has more
info so I'll close this as the dup.

*** This bug has been marked as a duplicate of bug 104091 ***