[committed] libstdc++: Make variant_npos conversions explicit [PR 96766]

[Jonathan Wakely via Gcc-patches]

libstdc++-v3/ChangeLog:

        PR libstdc++/96766
        * include/std/variant (_Variant_storage): Replace implicit
        conversions from size_t to __index_type with explicit casts.

This fixes a silly Clang UBsan warning from its silly
unsigned-integer-overflow sanitizer which complains about perfectly
valid code doing exactly what it's intended to. How silly.

Tested powerpc64le-linux, and with Clang. Committed to trunk.

I'll backport this to the branches, because people use clang's silly
sanitizer with the branches too.


commit 074436cf8cdd2a9ce75cadd36deb8301f00e55b9
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Mon Aug 24 16:10:07 2020

    libstdc++: Make variant_npos conversions explicit [PR 96766]
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/96766
            * include/std/variant (_Variant_storage): Replace implicit
            conversions from size_t to __index_type with explicit casts.

diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index eb3d6779205..dd8847cf829 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -385,12 +385,16 @@ namespace __variant
   template<typename... _Types>
     struct _Variant_storage<false, _Types...>
     {
-      constexpr _Variant_storage() : _M_index(variant_npos) { }
+      constexpr
+      _Variant_storage()
+      : _M_index(static_cast<__index_type>(variant_npos))
+      { }
 
       template<size_t _Np, typename... _Args>
-       constexpr _Variant_storage(in_place_index_t<_Np>, _Args&&... __args)
+       constexpr
+       _Variant_storage(in_place_index_t<_Np>, _Args&&... __args)
        : _M_u(in_place_index<_Np>, std::forward<_Args>(__args)...),
-       _M_index(_Np)
+       _M_index{_Np}
        { }
 
       void _M_reset()
@@ -403,7 +407,7 @@ namespace __variant
            std::_Destroy(std::__addressof(__this_mem));
          }, __variant_cast<_Types...>(*this));
 
-       _M_index = variant_npos;
+       _M_index = static_cast<__index_type>(variant_npos);
       }
 
       ~_Variant_storage()
@@ -432,16 +436,20 @@ namespace __variant
   template<typename... _Types>
     struct _Variant_storage<true, _Types...>
     {
-      constexpr _Variant_storage() : _M_index(variant_npos) { }
+      constexpr
+      _Variant_storage()
+      : _M_index(static_cast<__index_type>(variant_npos))
+      { }
 
       template<size_t _Np, typename... _Args>
-       constexpr _Variant_storage(in_place_index_t<_Np>, _Args&&... __args)
+       constexpr
+       _Variant_storage(in_place_index_t<_Np>, _Args&&... __args)
        : _M_u(in_place_index<_Np>, std::forward<_Args>(__args)...),
-       _M_index(_Np)
+       _M_index{_Np}
        { }
 
       void _M_reset() noexcept
-      { _M_index = variant_npos; }
+      { _M_index = static_cast<__index_type>(variant_npos); }
 
       void*
       _M_storage() const noexcept
@@ -455,7 +463,7 @@ namespace __variant
       {
        if constexpr (__variant::__never_valueless<_Types...>())
          return true;
-       return this->_M_index != __index_type(variant_npos);
+       return this->_M_index != static_cast<__index_type>(variant_npos);
       }
 
       _Variadic_union<_Types...> _M_u;