[gcc r12-4072] libstdc++: Fix narrowing conversion in std::visit

[Jonathan Wakely]

https://gcc.gnu.org/g:dc1b29508d765b874372f4b1737ac6dc86962506

commit r12-4072-gdc1b29508d765b874372f4b1737ac6dc86962506
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu May 20 18:04:16 2021 +0100

    libstdc++: Fix narrowing conversion in std::visit
    
    Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
    
    libstdc++-v3/ChangeLog:
    
            * include/std/variant (__do_visit): Use variant_npos instead of
            literal -1 that requires a narrowing conversion.

Diff:
---
 libstdc++-v3/include/std/variant | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index ddeefd9b35e..d50c6b7de1d 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -1830,7 +1830,8 @@ namespace __variant
 		  if constexpr (is_same_v<_Result_type, __variant_idx_cookie>
 				|| is_same_v<_Result_type, __variant_cookie>)
 		    {
-		      return __gen_vtable_impl<_Ma, index_sequence<-1>>::
+		      using _Npos = index_sequence<variant_npos>;
+		      return __gen_vtable_impl<_Ma, _Npos>::
 			__visit_invoke(std::forward<_Visitor>(__visitor),
 				       std::forward<_V0>(__v0));
 		    }