[libcxx-commits] [libcxx] Speed up compilation of common uses of std::visit() by ~8x (PR #164196)

[via libcxx-commits]

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@@ -1578,11 +1578,42 @@ _LIBCPP_HIDE_FROM_ABI constexpr void __throw_if_valueless(_Vs&&... __vs) {
   }
 }
 
-template < class _Visitor, class... _Vs, typename>
+template <class _Visitor, class... _Vs, typename>
 _LIBCPP_HIDE_FROM_ABI constexpr decltype(auto) visit(_Visitor&& __visitor, _Vs&&... __vs) {
-  using __variant_detail::__visitation::__variant;
-  std::__throw_if_valueless(std::forward<_Vs>(__vs)...);
-  return __variant::__visit_value(std::forward<_Visitor>(__visitor), std::forward<_Vs>(__vs)...);
+#    define _XDispatchIndex(_I)                                                                                        \
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higher-performance wrote:

Yeah, I deliberately avoided doing more work to make it easier to review and get feedback first. It adds edge cases to testing so I'm not sure how folks feel about it.

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