[Bug libstdc++/90943] Visiting inherited variants no longer works in 9.1

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90943

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c++                         |libstdc++

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This is all it takes to compile your example:

--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -795,7 +795,7 @@ namespace __variant
   template <typename _Maybe_variant_cookie, typename _Variant>
     struct _Extra_visit_slot_needed
     {
-      template <typename> struct _Variant_never_valueless;
+      template <typename> struct _Variant_never_valueless : false_type { };

       template <typename... _Types>
        struct _Variant_never_valueless<variant<_Types...>>

This just assumes that unrecognized types are not never-valueless variants,
which means some extra code gets generated to handle the valueless case even if
your variant base class will never be valueless.

I haven't considered if anything else would break by doing that.