[libcxx] adds common_reference to <type_traits>

Implements part of P0898R3 Standard Library Concepts

Reworks D74351 to use requires-clauses over SFINAE and so that it more
closely follows the wording.

Co-authored by: Michael Schellenberger Costa <[email protected]>

(Michael did all the heavy lifting and I came in to polish it for
 submission, since Michael is focussing on `std::format` now.)

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D96657

Author: cjdb

libcxx/include/type_traits

@@ -2411,6 +2411,216 @@ struct _LIBCPP_TEMPLATE_VIS
 template <class ..._Tp> using common_type_t = typename common_type<_Tp...>::type;
 #endif
 
+#if _LIBCPP_STD_VER > 11
+// Let COPYCV(FROM, TO) be an alias for type TO with the addition of FROM’s
+// top-level cv-qualifiers.
+template <class _From, class _To>
+struct __copy_cv
+{
+    using type = _To;
+};
+
+template <class _From, class _To>
+struct __copy_cv<const _From, _To>
+{
+    using type = add_const_t<_To>;
+};
+
+template <class _From, class _To>
+struct __copy_cv<volatile _From, _To>
+{
+    using type = add_volatile_t<_To>;
+};
+
+template <class _From, class _To>
+struct __copy_cv<const volatile _From, _To>
+{
+    using type = add_cv_t<_To>;
+};
+
+template <class _From, class _To>
+using __copy_cv_t = typename __copy_cv<_From, _To>::type;
+
+template <class _From, class _To>
+struct __copy_cvref
+{
+    using type = __copy_cv_t<_From, _To>;
+};
+
+template <class _From, class _To>
+struct __copy_cvref<_From&, _To>
+{
+    using type = add_lvalue_reference_t<__copy_cv_t<_From, _To>>;
+};
+
+template <class _From, class _To>
+struct __copy_cvref<_From&&, _To>
+{
+    using type = add_rvalue_reference_t<__copy_cv_t<_From, _To>>;
+};
+
+template <class _From, class _To>
+using __copy_cvref_t = typename __copy_cvref<_From, _To>::type;
+
+#endif // _LIBCPP_STD_VER > 11
+
+// common_reference
+#if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_CONCEPTS)
+// Let COND_RES(X, Y) be:
+template <class _Xp, class _Yp>
+using __cond_res =
+    decltype(false ? _VSTD::declval<_Xp(&)()>()() : _VSTD::declval<_Yp(&)()>()());
+
+// Let `XREF(A)` denote a unary alias template `T` such that `T<U>` denotes the same type as `U`
+// with the addition of `A`'s cv and reference qualifiers, for a non-reference cv-unqualified type
+// `U`.
+// [Note: `XREF(A)` is `__xref<A>::template __apply`]
+template <class _Tp>
+struct __xref {
+  template<class _Up>
+  using __apply = __copy_cvref_t<_Tp, _Up>;
+};
+
+// Given types `A` and `B`, let `X` be `remove_­reference_­t<A>`, let `Y` be `remove_­reference_­t<B>`,
+// and let `COMMON-​REF(A, B)` be:
+template<class _Ap, class _Bp, class _Xp = remove_reference_t<_Ap>, class _Yp = remove_reference_t<_Bp>>
+struct __common_ref;
+
+template<class _Xp, class _Yp>
+using __common_ref_t = typename __common_ref<_Xp, _Yp>::__type;
+
+template<class _Xp, class _Yp>
+using __cv_cond_res = __cond_res<__copy_cv_t<_Xp, _Yp>&, __copy_cv_t<_Yp, _Xp>&>;
+
+
+//    If `A` and `B` are both lvalue reference types, `COMMON-REF(A, B)` is
+//    `COND-RES(COPYCV(X, Y) &, COPYCV(​Y, X) &)` if that type exists and is a reference type.
+template<class _Ap, class _Bp, class _Xp, class _Yp>
+requires requires { typename __cv_cond_res<_Xp, _Yp>; } && is_reference_v<__cv_cond_res<_Xp, _Yp>>
+struct __common_ref<_Ap&, _Bp&, _Xp, _Yp>
+{
+    using __type = __cv_cond_res<_Xp, _Yp>;
+};
+
+//    Otherwise, let `C` be `remove_­reference_­t<COMMON-REF(X&, Y&)>&&`....
+template <class _Xp, class _Yp>
+using __common_ref_C = remove_reference_t<__common_ref_t<_Xp&, _Yp&>>&&;
+
+
+//    .... If `A` and `B` are both rvalue reference types, `C` is well-formed, and
+//    `is_­convertible_­v<A, C> && is_­convertible_­v<B, C>` is `true`, then `COMMON-REF(A, B)` is `C`.
+template<class _Ap, class _Bp, class _Xp, class _Yp>
+requires
+  requires { typename __common_ref_C<_Xp, _Yp>; } &&
+  is_convertible_v<_Ap&&, __common_ref_C<_Xp, _Yp>> &&
+  is_convertible_v<_Bp&&, __common_ref_C<_Xp, _Yp>>
+struct __common_ref<_Ap&&, _Bp&&, _Xp, _Yp>
+{
+    using __type = __common_ref_C<_Xp, _Yp>;
+};
+
+//    Otherwise, let `D` be `COMMON-REF(const X&, Y&)`....
+template <class _Tp, class _Up>
+using __common_ref_D = __common_ref_t<const _Tp&, _Up&>;
+
+//    ... If `A` is an rvalue reference and `B` is an lvalue reference and `D` is well-formed and
+//    `is_­convertible_­v<A, D>` is `true`, then `COMMON-REF(A, B)` is `D`.
+template<class _Ap, class _Bp, class _Xp, class _Yp>
+requires requires { typename __common_ref_D<_Xp, _Yp>; } &&
+         is_convertible_v<_Ap&&, __common_ref_D<_Xp, _Yp>>
+struct __common_ref<_Ap&&, _Bp&, _Xp, _Yp>
+{
+    using __type = __common_ref_D<_Xp, _Yp>;
+};
+
+//    Otherwise, if `A` is an lvalue reference and `B` is an rvalue reference, then
+//    `COMMON-REF(A, B)` is `COMMON-REF(B, A)`.
+template<class _Ap, class _Bp, class _Xp, class _Yp>
+struct __common_ref<_Ap&, _Bp&&, _Xp, _Yp> : __common_ref<_Bp&&, _Ap&> {};
+
+//    Otherwise, `COMMON-REF(A, B)` is ill-formed.
+template<class _Ap, class _Bp, class _Xp, class _Yp>
+struct __common_ref {};
+
+// Note C: For the common_reference trait applied to a parameter pack [...]
+
+template <class...>
+struct common_reference;
+
+template <class... _Types>
+using common_reference_t = typename common_reference<_Types...>::type;
+
+// bullet 1 - sizeof...(T) == 0
+template<>
+struct common_reference<> {};
+
+// bullet 2 - sizeof...(T) == 1
+template <class _Tp>
+struct common_reference<_Tp>
+{
+    using type = _Tp;
+};
+
+// bullet 3 - sizeof...(T) == 2
+template <class _Tp, class _Up> struct __common_reference_sub_bullet3;
+template <class _Tp, class _Up> struct __common_reference_sub_bullet2 : __common_reference_sub_bullet3<_Tp, _Up> {};
+template <class _Tp, class _Up> struct __common_reference_sub_bullet1 : __common_reference_sub_bullet2<_Tp, _Up> {};
+
+// sub-bullet 1 - If `T1` and `T2` are reference types and `COMMON-REF(T1, T2)` is well-formed, then
+// the member typedef type denotes that type.
+template <class _Tp, class _Up> struct common_reference<_Tp, _Up> : __common_reference_sub_bullet1<_Tp, _Up> {};
+
+template <class _Tp, class _Up>
+requires is_reference_v<_Tp> && is_reference_v<_Up> && requires { typename __common_ref_t<_Tp, _Up>; }
+struct __common_reference_sub_bullet1<_Tp, _Up>
+{
+    using type = __common_ref_t<_Tp, _Up>;
+};
+
+// sub-bullet 2 - Otherwise, if `basic_­common_­reference<remove_­cvref_­t<T1>, remove_­cvref_­t<T2>, ​XREF(​T1), XREF(T2)>​::​type`
+// is well-formed, then the member typedef type denotes that type.
+template <class, class, template <class> class, template <class> class> struct basic_common_reference {};
+
+template <class _Tp, class _Up>
+using __basic_common_reference_t = typename basic_common_reference<
+    remove_cvref_t<_Tp>, remove_cvref_t<_Up>,
+    __xref<_Tp>::template __apply, __xref<_Up>::template __apply>::type;
+
+template <class _Tp, class _Up>
+requires requires { typename __basic_common_reference_t<_Tp, _Up>; }
+struct __common_reference_sub_bullet2<_Tp, _Up>
+{
+    using type = __basic_common_reference_t<_Tp, _Up>;
+};
+
+// sub-bullet 3 - Otherwise, if `COND-RES(T1, T2)` is well-formed, then the member typedef type
+// denotes that type.
+template <class _Tp, class _Up>
+requires requires { typename __cond_res<_Tp, _Up>; }
+struct __common_reference_sub_bullet3<_Tp, _Up>
+{
+    using type = __cond_res<_Tp, _Up>;
+};
+
+
+// sub-bullet 4 & 5 - Otherwise, if `common_­type_­t<T1, T2>` is well-formed, then the member typedef
+//                    type denotes that type.
+//                  - Otherwise, there shall be no member type.
+template <class _Tp, class _Up> struct __common_reference_sub_bullet3 : common_type<_Tp, _Up> {};
+
+// bullet 4 - If there is such a type `C`, the member typedef type shall denote the same type, if
+//            any, as `common_­reference_­t<C, Rest...>`.
+template <class _Tp, class _Up, class _Vp, class... _Rest>
+requires requires { typename common_reference_t<_Tp, _Up>; }
+struct common_reference<_Tp, _Up, _Vp, _Rest...>
+    : common_reference<common_reference_t<_Tp, _Up>, _Vp, _Rest...>
+{};
+
+// bullet 5 - Otherwise, there shall be no member type.
+template <class...> struct common_reference {};
+
+#endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_CONCEPTS)
+
 // is_assignable
 
 template<typename, typename _Tp> struct __select_2nd { typedef _LIBCPP_NODEBUG_TYPE _Tp type; };