[Bug c++/106051] New: -Wmismatched-tags gives false positive warning when "class C" is "using namespace"d into a namespace with another "class C" declaration

[beardsley.matt.j at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106051

            Bug ID: 106051
           Summary: -Wmismatched-tags gives false positive warning when
                    "class C" is "using namespace"d into a namespace with
                    another "class C" declaration
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: beardsley.matt.j at gmail dot com
  Target Milestone: ---

https://godbolt.org/z/sx8aT3EKh



The following uses gcc built from a fresh repo that I just cloned earlier today
(default build settings)

tmp.cpp:
  namespace ns {
  class C {};
  }
  using namespace ns;
  class C;

No ambiguity with the warning off:
$ g++ -c tmp.cpp -o /dev/null -Wno-mismatched-tags
(no output)

Now it's ambiguous (also refers to it as "struct C" but it's really "class C")
$ g++ -c tmp.cpp -o /dev/null -Wmismatched-tags
tmp.cpp:5:7: error: reference to ‘C’ is ambiguous
    5 | class C;
      |       ^
tmp.cpp:2:7: note: candidates are: ‘class ns::C’
    2 | class C {};
      |       ^
tmp.cpp:5:7: note:                 ‘struct C’
    5 | class C;
      |       ^


Not sure how noteworthy this is but if the last 2 lines are swapped the warning
goes away:

tmp.cpp:
  namespace ns {
  class C {};
  }
  class C;
  using namespace ns;

$ g++ -c tmp.cpp -o /dev/null -Wno-mismatched-tags
(no output)

$ g++ -c tmp.cpp -o /dev/null -Wmismatched-tags
(no output)