[gcc r12-4419] libstdc++: Simplify variant access functions

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:4f87d4c5aec9a1eaca7be61e5c8aab4d6e61b1d8

commit r12-4419-g4f87d4c5aec9a1eaca7be61e5c8aab4d6e61b1d8
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Oct 14 20:37:38 2021 +0100

    libstdc++: Simplify variant access functions
    
    libstdc++-v3/ChangeLog:
    
            * include/std/variant (__variant::__get(in_place_index_t<N>, U&&)):
            Rename to __get_n and remove first argument. Replace pair of
            overloads with a single function using 'if constexpr'.
            (__variant::__get(Variant&&)): Adjust to use __get_n.

Diff:
---
 libstdc++-v3/include/std/variant | 23 +++++++++++------------
 1 file changed, 11 insertions(+), 12 deletions(-)

diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index 6377b6731ea..b85a89d0b7b 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -279,27 +279,26 @@ namespace __variant
       __gnu_cxx::__aligned_membuf<_Type> _M_storage;
     };
 
-  template<typename _Union>
-    constexpr decltype(auto)
-    __get(in_place_index_t<0>, _Union&& __u) noexcept
-    { return std::forward<_Union>(__u)._M_first._M_get(); }
-
   template<size_t _Np, typename _Union>
     constexpr decltype(auto)
-    __get(in_place_index_t<_Np>, _Union&& __u) noexcept
+    __get_n(_Union&& __u) noexcept
     {
-      return __variant::__get(in_place_index<_Np-1>,
-			      std::forward<_Union>(__u)._M_rest);
+      if constexpr (_Np == 0)
+	return std::forward<_Union>(__u)._M_first._M_get();
+      else if constexpr (_Np == 1)
+	return std::forward<_Union>(__u)._M_rest._M_first._M_get();
+      else if constexpr (_Np == 2)
+	return std::forward<_Union>(__u)._M_rest._M_rest._M_first._M_get();
+      else
+	return __variant::__get_n<_Np - 3>(
+		 std::forward<_Union>(__u)._M_rest._M_rest._M_rest);
     }
 
   // Returns the typed storage for __v.
   template<size_t _Np, typename _Variant>
     constexpr decltype(auto)
     __get(_Variant&& __v) noexcept
-    {
-      return __variant::__get(std::in_place_index<_Np>,
-			      std::forward<_Variant>(__v)._M_u);
-    }
+    { return __variant::__get_n<_Np>(std::forward<_Variant>(__v)._M_u); }
 
   template<typename... _Types>
     struct _Traits