[gcc r12-5637] libstdc++: Skip tag dispatching for _S_relocate in C++17

[gcc r12-5637] libstdc++: Skip tag dispatching for _S_relocate in C++17

[Jonathan Wakely]

https://gcc.gnu.org/g:91c26004037db689954318d3d1c801eea18d45f4

commit r12-5637-g91c26004037db689954318d3d1c801eea18d45f4
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Nov 30 13:41:32 2021 +0000

    libstdc++: Skip tag dispatching for _S_relocate in C++17
    
    In C++17 mode all callers of _S_relocate have already done:
    
      if constexpr (_S_use_relocate())
    
    so we don't need to repeat that check and use tag dispatching to avoid
    ill-formed instantiations.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/stl_vector.h (vector::_S_do_relocate): Remove
            C++20 constexpr specifier.
            (vector::_S_relocate) [__cpp_if_constexpr]: Call __relocate_a
            directly without tag dispatching.

Diff:
---
 libstdc++-v3/include/bits/stl_vector.h | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/bits/stl_vector.h b/libstdc++-v3/include/bits/stl_vector.h
index 4587757637e..36b2cff3d78 100644
--- a/libstdc++-v3/include/bits/stl_vector.h
+++ b/libstdc++-v3/include/bits/stl_vector.h
@@ -481,14 +481,14 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
 	return _S_nothrow_relocate(__is_move_insertable<_Tp_alloc_type>{});
       }
 
-      static _GLIBCXX20_CONSTEXPR pointer
+      static pointer
       _S_do_relocate(pointer __first, pointer __last, pointer __result,
 		     _Tp_alloc_type& __alloc, true_type) noexcept
       {
 	return std::__relocate_a(__first, __last, __result, __alloc);
       }
 
-      static _GLIBCXX20_CONSTEXPR pointer
+      static pointer
       _S_do_relocate(pointer, pointer, pointer __result,
 		     _Tp_alloc_type&, false_type) noexcept
       { return __result; }
@@ -497,8 +497,13 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
       _S_relocate(pointer __first, pointer __last, pointer __result,
 		  _Tp_alloc_type& __alloc) noexcept
       {
+#if __cpp_if_constexpr
+	// All callers have already checked _S_use_relocate() so just do it.
+	return std::__relocate_a(__first, __last, __result, __alloc);
+#else
 	using __do_it = __bool_constant<_S_use_relocate()>;
 	return _S_do_relocate(__first, __last, __result, __alloc, __do_it{});
+#endif
       }
 #endif // C++11