[libcxx-commits] [libcxx] Speed up compilation of common uses of std::visit() (PR #164196)

[via libcxx-commits]

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@@ -1332,6 +1332,10 @@ private:
   friend struct __variant_detail::__visitation::__variant;
 };
 
+template <class... _Types>
+variant<_Types...> __upcast_to_variant(const volatile variant<_Types...>*);
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higher-performance wrote:

I think the upcast is important. Otherwise, you end up with `__vs.__first_variant_type::index()` being called. But in that case `__first_variant_type` is not necessarily a `std::variant`; it might be a derived class, causing the wrong `index()` method to get called. I'll add a comment to clarify.

https://github.com/llvm/llvm-project/pull/164196