[gcc r11-7339] libstdc++: Fix up parallel_backend_serial.h [PR97549]

[Jakub Jelinek <jakub@gcc.gnu.org>]

https://gcc.gnu.org/g:7e647d71d556b73e84f4edd0733bcc83bc70ae1e

commit r11-7339-g7e647d71d556b73e84f4edd0733bcc83bc70ae1e
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue Feb 23 09:26:23 2021 +0100

    libstdc++: Fix up parallel_backend_serial.h [PR97549]
    
    In GCC 10, parallel_backend.h just included parallel_backend_{serial,tbb}.h and
    did nothing beyond that, and parallel_backend_tbb.h provided directly
    namespace __pstl { namespace __par_backend { ... } }
    and defined everything in there, while parallel_backend_serial.h did:
    namespace __pstl { namespace __serial { ... } } and had this
    namespace __pstl { namespace __par_backend { using namespace __pstl::__serial; } }
    at the end.
    In GCC 11, parallel_backend.h does:
    namespace __pstl { namespace __par_backend = __serial_backend; }
    after including parallel_backend_serial.h or
    namespace __pstl { namespace __par_backend = __tbb_backend; }
    after including parallel_backend_tbb.h.  The latter then has:
    namespace __pstl { namespace __tbb_backend { ... } }
    and no using etc. at the end, while parallel_backend_serial.h changed to:
    namespace __pstl { namespace __serial_backend { ... } }
    but has this leftover block from the GCC 10 times.  Even changing that
    using namespace __pstl::__serial;
    to
    using namespace __pstl::__serial_backend;
    doesn't work, as it clashes with
    namespace __pstl { namespace __par_backend = __serial_backend; }
    in parallel_backend.h.
    
    2021-02-23  Jakub Jelinek  <jakub@redhat.com>
    
            PR libstdc++/97549
            * include/pstl/parallel_backend_serial.h: Remove __pstl::__par_backend.

Diff:
---
 libstdc++-v3/include/pstl/parallel_backend_serial.h | 8 --------
 1 file changed, 8 deletions(-)

diff --git a/libstdc++-v3/include/pstl/parallel_backend_serial.h b/libstdc++-v3/include/pstl/parallel_backend_serial.h
index 32abe77321e..16e773d411a 100644
--- a/libstdc++-v3/include/pstl/parallel_backend_serial.h
+++ b/libstdc++-v3/include/pstl/parallel_backend_serial.h
@@ -127,12 +127,4 @@ __parallel_invoke(_ExecutionPolicy&&, _F1&& __f1, _F2&& __f2)
 } // namespace __serial_backend
 } // namespace __pstl
 
-namespace __pstl
-{
-namespace __par_backend
-{
-using namespace __pstl::__serial;
-}
-} // namespace __pstl
-
 #endif /* _PSTL_PARALLEL_BACKEND_SERIAL_H */