libstdc++: Add std::reference_wrapper comparison operators for C++26

This C++26 change was just approved in Tokyo, in P2944R3. It adds
operator== and operator<=> overloads to std::reference_wrapper.

The operator<=> overloads in the paper cause compilation errors for any
type without <=> so they're implemented here with deduced return types
and constrained by a requires clause.

libstdc++-v3/ChangeLog:

	* include/bits/refwrap.h (reference_wrapper): Add comparison
	operators as proposed by P2944R3.
	* include/bits/version.def (reference_wrapper): Define.
	* include/bits/version.h: Regenerate.
	* include/std/functional: Enable feature test macro.
	* testsuite/20_util/reference_wrapper/compare.cc: New test.

5 files changed, 159 insertions, 0 deletions

diff --git a/libstdc++-v3/include/bits/refwrap.h b/libstdc++-v3/include/bits/refwrap.h
index 2d4338b..fd1cc2b 100644
--- a/libstdc++-v3/include/bits/refwrap.h
+++ b/libstdc++-v3/include/bits/refwrap.h
@@ -38,6 +38,10 @@
 #include <bits/invoke.h>
 #include <bits/stl_function.h> // for unary_function and binary_function
 
+#if __glibcxx_reference_wrapper >= 202403L // >= C++26
+# include <compare>
+#endif
+
 namespace std _GLIBCXX_VISIBILITY(default)
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
@@ -358,6 +362,47 @@ _GLIBCXX_MEM_FN_TRAITS(&& noexcept, false_type, true_type)
 #endif
 	  return std::__invoke(get(), std::forward<_Args>(__args)...);
 	}
+
+#if __glibcxx_reference_wrapper >= 202403L // >= C++26
+      // [refwrap.comparisons], comparisons
+      [[nodiscard]]
+      friend constexpr bool
+      operator==(reference_wrapper __x, reference_wrapper __y)
+      requires requires { { __x.get() == __y.get() } -> convertible_to<bool>; }
+      { return __x.get() == __y.get(); }
+
+      [[nodiscard]]
+      friend constexpr bool
+      operator==(reference_wrapper __x, const _Tp& __y)
+      requires requires { { __x.get() == __y } -> convertible_to<bool>; }
+      { return __x.get() == __y; }
+
+      [[nodiscard]]
+      friend constexpr bool
+      operator==(reference_wrapper __x, reference_wrapper<const _Tp> __y)
+      requires (!is_const_v<_Tp>)
+	&& requires { { __x.get() == __y.get() } -> convertible_to<bool>; }
+      { return __x.get() == __y.get(); }
+
+      [[nodiscard]]
+      friend constexpr auto
+      operator<=>(reference_wrapper __x, reference_wrapper<_Tp> __y)
+      requires requires { __detail::__synth3way(__x.get(), __y.get()); }
+      { return __detail::__synth3way(__x.get(), __y.get()); }
+
+      [[nodiscard]]
+      friend constexpr auto
+      operator<=>(reference_wrapper __x, const _Tp& __y)
+      requires requires { __detail::__synth3way(__x.get(), __y); }
+      { return __detail::__synth3way(__x.get(), __y); }
+
+      [[nodiscard]]
+      friend constexpr auto
+      operator<=>(reference_wrapper __x, reference_wrapper<const _Tp> __y)
+      requires (!is_const_v<_Tp>)
+	&& requires { __detail::__synth3way(__x.get(), __y.get()); }
+      { return __detail::__synth3way(__x.get(), __y.get()); }
+#endif
     };
 
 #if __cpp_deduction_guides
diff --git a/libstdc++-v3/include/bits/version.def b/libstdc++-v3/include/bits/version.def
index 5ad4494..5c0477f 100644
--- a/libstdc++-v3/include/bits/version.def
+++ b/libstdc++-v3/include/bits/version.def
@@ -1761,6 +1761,14 @@ ftms = {
 };
 
 ftms = {
+  name = reference_wrapper;
+  values = {
+    v = 202403;
+    cxxmin = 26;
+  };
+};
+
+ftms = {
   name = saturation_arithmetic;
   values = {
     v = 202311;
diff --git a/libstdc++-v3/include/bits/version.h b/libstdc++-v3/include/bits/version.h
index 460a3e0..65e708c 100644
--- a/libstdc++-v3/include/bits/version.h
+++ b/libstdc++-v3/include/bits/version.h
@@ -1963,6 +1963,16 @@
 #endif /* !defined(__cpp_lib_ratio) && defined(__glibcxx_want_ratio) */
 #undef __glibcxx_want_ratio
 
+#if !defined(__cpp_lib_reference_wrapper)
+# if (__cplusplus >  202302L)
+#  define __glibcxx_reference_wrapper 202403L
+#  if defined(__glibcxx_want_all) || defined(__glibcxx_want_reference_wrapper)
+#   define __cpp_lib_reference_wrapper 202403L
+#  endif
+# endif
+#endif /* !defined(__cpp_lib_reference_wrapper) && defined(__glibcxx_want_reference_wrapper) */
+#undef __glibcxx_want_reference_wrapper
+
 #if !defined(__cpp_lib_saturation_arithmetic)
 # if (__cplusplus >  202302L)
 #  define __glibcxx_saturation_arithmetic 202311L
diff --git a/libstdc++-v3/include/std/functional b/libstdc++-v3/include/std/functional
index 766558b..9936428 100644
--- a/libstdc++-v3/include/std/functional
+++ b/libstdc++-v3/include/std/functional
@@ -83,6 +83,7 @@
 #define __glibcxx_want_move_only_function
 #define __glibcxx_want_not_fn
 #define __glibcxx_want_ranges
+#define __glibcxx_want_reference_wrapper
 #define __glibcxx_want_transparent_operators
 #include <bits/version.h>
 
diff --git a/libstdc++-v3/testsuite/20_util/reference_wrapper/compare.cc b/libstdc++-v3/testsuite/20_util/reference_wrapper/compare.cc
new file mode 100644
index 0000000..039c9d2
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/reference_wrapper/compare.cc
@@ -0,0 +1,95 @@
+// { dg-do compile { target c++26 } }
+
+
+#include <functional>
+
+#ifndef __cpp_lib_reference_wrapper
+# error "Feature-test macro for reference_wrapper missing"
+#elif __cpp_lib_reference_wrapper != 202403
+# error "Feature-test macro for reference_wrapper has wrong value"
+#endif
+
+// P2944R3 Comparisons for reference_wrapper
+
+auto check(int i, std::reference_wrapper<int> r) -> bool {
+  return i == r;
+}
+
+template <class T> using Ref = std::reference_wrapper<T>;
+
+template <class T>
+concept ref_equality_comparable
+= requires (T a, T const ca, Ref<T> r, Ref<T const> cr) {
+    // the usual T is equality-comparable with itself
+    a == a;
+    a == ca;
+    ca == ca;
+
+    // Ref<T> is equality-comparable with itself
+    r == r;
+    r == cr;
+    cr == cr;
+
+    // T and Ref<T> are equality-comparable
+    a == r;
+    a == cr;
+    ca == r;
+    ca == cr;
+};
+
+static_assert( ref_equality_comparable<int> );
+
+struct A {
+    auto operator==(A const&) const -> bool { return true; }
+};
+
+struct B {
+    friend auto operator==(B const&, B const&) -> bool { return true; }
+};
+
+template <class T>
+struct C {
+    friend auto operator==(C const&, C const&) -> bool { return true; }
+};
+
+template <class T>
+struct D { };
+template <class T>
+auto operator==(D<T> const&, D<T> const&) -> bool { return true; }
+
+static_assert(ref_equality_comparable<int>);
+static_assert(ref_equality_comparable<A>);
+static_assert(ref_equality_comparable<B>);
+static_assert(ref_equality_comparable<C<int>>);
+static_assert(ref_equality_comparable<D<int>>);
+#include <string_view>
+static_assert(ref_equality_comparable<std::string_view>);
+
+template <typename T>
+struct ValArray {
+  friend auto operator==(ValArray const&, ValArray const&) -> ValArray<bool> {
+    return {};
+  }
+};
+
+void f(ValArray<int> v) {
+  // this is valid and has type ValArray<bool>
+  v == v;
+
+  // this is also valid today and has the same type
+  std::ref(v) == std::ref(v);
+}
+
+struct ComparesAsInt {
+  friend auto operator==(ComparesAsInt, ComparesAsInt) -> int;
+};
+
+auto f(std::reference_wrapper<ComparesAsInt> a,
+    std::reference_wrapper<ComparesAsInt> b) {
+  // today: compiles and returns int
+  // proposed: compiles and returns bool
+  return a == b;
+}
+
+ComparesAsInt& c();
+static_assert( std::is_same_v<decltype(f(c(), c())), bool> );