[libcxx-commits] [libcxx] [libc++] Optimize std::swap of locales (PR #209760)

[A. Jiang via libcxx-commits]

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@@ -143,8 +143,14 @@ private:
   friend bool has_facet(const locale&) _NOEXCEPT;
   template <class _Facet>
   friend const _Facet& use_facet(const locale&);
+
+  friend void swap(locale&, locale&);
 };
 
+inline void swap(locale& __lhs, locale& __rhs) {
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frederick-vs-ja wrote:

> The standard doesn't allow for a move constructor.

I think the current approach is already slightly non-conforming as it slightly changes the behavior for `std::swap` in overload resolution. See [this reduced example](https://godbolt.org/z/e3vqGsdfK).

Perhaps we should submit an LWG issue to add missing move functions to `locale`.

https://github.com/llvm/llvm-project/pull/209760