Add workaround to std::variant for Clang bug 31852

[Jonathan Wakely]

On 26 March 2018 at 14:10, Jonathan Wakely wrote:
> This makes it possible to use our std::variant with Clang, as well as
> some minor tweaks to avoid ADL (so the compiler doesn't waste time
> looking in associated namespaces) and adjust whitespace.
>
>        * include/std/variant (__get): Qualify calls to avoid ADL.
>        (__select_index): Adjust whitespace.
>        (variant): Add using-declaration to workaround Clang bug.
>
> Tested powerpc64le-linux, committed to trunk.. I'll backport to
> gcc-7-branch too.

The Clang bug means that __get is ambiguous (because it thinks the
friend declaration is a separate overload) so we also need this
change.

Tested powerpc64le-linux, committed to trunk.
-------------- next part --------------
commit c09f881053d98370306098b7bad5bb829255379c
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Apr 5 14:03:13 2018 +0100

    Add another workaround to std::variant for Clang bug 31852
    
            * include/std/variant (_VARIANT_RELATION_FUNCTION_TEMPLATE): Qualify
            __get calls to avoid ADL and avoid ambiguity due to Clang bug.

diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index 028d3064272..48bec528406 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -272,8 +272,8 @@ namespace __variant
     constexpr bool \
     __erased_##__NAME(const _Variant& __lhs, const _Variant& __rhs) \
     { \
-      return __get<_Np>(std::forward<_Variant>(__lhs)) \
-	  __OP __get<_Np>(std::forward<_Variant>(__rhs)); \
+      return __variant::__get<_Np>(std::forward<_Variant>(__lhs)) \
+	  __OP __variant::__get<_Np>(std::forward<_Variant>(__rhs)); \
     }
 
   _VARIANT_RELATION_FUNCTION_TEMPLATE(<, less)