[libcxx-commits] [libcxx] [libc++] Make lazy three way comparator optimization apply in older standard modes (PR #218525)

[via libcxx-commits]

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@@ -713,16 +714,16 @@ template <class _MapValueT, class _Key, class _TransparentKey, class _Compare>
 struct __lazy_synth_three_way_comparator<__map_value_compare<_Key, _MapValueT, _Compare>, _MapValueT, _TransparentKey> {
   __lazy_synth_three_way_comparator<_Compare, _Key, _TransparentKey> __comp_;
 
-  __lazy_synth_three_way_comparator(
+  _LIBCPP_CONSTEXPR_SINCE_CXX26 __lazy_synth_three_way_comparator(
       _LIBCPP_CTOR_LIFETIMEBOUND const __map_value_compare<_Key, _MapValueT, _Compare>& __comp)
       : __comp_(__comp.key_comp()) {}
 
-  _LIBCPP_HIDE_FROM_ABI auto
-  operator()(_LIBCPP_LIFETIMEBOUND const _MapValueT& __lhs, _LIBCPP_LIFETIMEBOUND const _TransparentKey& __rhs) const {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 auto
+  operator()(_LIBCPP_LIFETIMEBOUND const _MapValueT& __lhs, _LIBCPP_LIFETIMEBOUND const _TransparentKey& __rhs) const
+      -> decltype(__comp_(__lhs.first, __rhs)) {
----------------
zhangweize9-cyber wrote:

> I guess we may want to make this compile in C++03 mode, although the old style is really verbose.
> ```suggestion
>   _LIBCPP_HIDE_FROM_ABI
>   _LIBCPP_CONSTEXPR_SINCE_CXX26 decltype(std::declval<const __lazy_synth_three_way_comparator<_Compare, _Key, _Key>&>()(
>       std::declval<const _MapValueT&>().first, std::declval<const _TransparentKey&>()))
>   operator()(_LIBCPP_LIFETIMEBOUND const _MapValueT& __lhs, _LIBCPP_LIFETIMEBOUND const _TransparentKey& __rhs) const {
> ```
> 
> (Same above. We should also include `<__utility/declval.h>` for this.)

I don't recommend making this change. `_LIBCPP_HIDE_FROM_ABI` is an ABI hiding macro simply to hide ABI symbols and prevent dynamic library conflicts.

https://github.com/llvm/llvm-project/pull/218525