[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.