[Bug c++/95875] New: Misleading error message "invalid use of incomplete type"

[haoxintu at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95875

            Bug ID: 95875
           Summary: Misleading error message "invalid use of incomplete
                    type"
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: haoxintu at gmail dot com
                CC: mpolacek at gcc dot gnu.org
  Target Milestone: ---

$cat bug.cc
class A: 
public 
A,
A,
A
{} g_class;

$g++ -c bug.cc
bug.cc:5:1: error: invalid use of incomplete type ‘class A’
    5 | A
      | ^
bug.cc:1:7: note: forward declaration of ‘class A’
    1 | class A:
      |       ^
bug.cc:5:1: error: invalid use of incomplete type ‘class A’
    5 | A
      | ^
bug.cc:1:7: note: forward declaration of ‘class A’
    1 | class A:
      |       ^
bug.cc:5:1: error: invalid use of incomplete type ‘class A’
    5 | A
      | ^
bug.cc:1:7: note: forward declaration of ‘class A’
    1 | class A:
      |       ^

While in clang
$clang++ -c bug.cc
bug.cc:3:1: error: base class has incomplete type
A,
^
bug.cc:1:7: note: definition of 'A' is not complete until the closing '}'
class A: 
      ^
1 error generated.

I doubt that how GCC deals with this case. I guess there might be two
situations:
1. GCC gives the wrong line number.
2. GCC just emits the duplicated messages.

I guess GCC might have the second issue. I have tested in almost all GCC
versions, and they all have this issue.