[Bug c++/95809] New: GCC treats inline namespace declaration as "ambiguous"

["haoxintu at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95809

            Bug ID: 95809
           Summary: GCC treats inline namespace declaration as "ambiguous"
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: haoxintu at gmail dot com
  Target Milestone: ---

This code, bug.cc, GCC fails to compile it, while clang, icc, or msvc accepts
it well.

$cat bug.cc
namespace g_namespace {}
inline namespace {
    namespace g_namespace {}
}
namespace g_namespace {}

$g++ -c bug.cc
bug.cc:5:12: error: ‘namespace g_namespace’ is ambiguous
    5 | namespace  g_namespace {}
      |            ^~~~~~~~~~~
bug.cc:3:15: note: candidates are: ‘namespace <unnamed>::g_namespace { }’
    3 |     namespace g_namespace {}
      |               ^~~~~~~~~~~
bug.cc:1:11: note:                 ‘namespace g_namespace { }’
    1 | namespace g_namespace {}
      |           ^~~~~~~~~~~

My GCC version is
g++ (GCC) 11.0.0 20200605 (experimental)
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This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.


When I remove the keyword "inline", GCC accepts it well. So, I guess GCC might
can not detect the inline namespace declaration and separate its inside
namespace from outside ones.