Our implementation of swappable_with speculatively implemented the proposed resolution for LWG-3175 (a Library Issue, which is a bug report against the C++ Standard):
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template <class _Ty1, class _Ty2>
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concept swappable_with =
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#if 1 // Implement the PR of LWG-3175
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common_reference_with<_Ty1, _Ty2>
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#else // ^^^ LWG-3175 / N4810 vvv
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common_reference_with<const remove_reference_t<_Ty1>&, const remove_reference_t<_Ty2>&>
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#endif // select LWG-3175 vs. N4810
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&& requires(_Ty1&& __t, _Ty2&& __u) {
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_RANGES swap(static_cast<_Ty1&&>(__t), static_cast<_Ty1&&>(__t));
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_RANGES swap(static_cast<_Ty2&&>(__u), static_cast<_Ty2&&>(__u));
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_RANGES swap(static_cast<_Ty1&&>(__t), static_cast<_Ty2&&>(__u));
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_RANGES swap(static_cast<_Ty2&&>(__u), static_cast<_Ty1&&>(__t));
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};
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At the February 2020 meeting in Prague, LWG-3175 was resolved for C++20. Therefore, we should keep line 186 here, but remove lines 185 and 187-189.