libstdc++: Heterogeneous std::pair comparisons [PR113386]

I'm only treating this as a DR for C++20 for now, because it's less work
and only requires changes to operator== and operator<=>. To do this for
older standards would require changes to the six relational operators
used pre-C++20.

libstdc++-v3/ChangeLog:

	PR libstdc++/113386
	* include/bits/stl_pair.h (operator==, operator<=>): Support
	heterogeneous comparisons, as per LWG 3865.
	* testsuite/20_util/pair/comparison_operators/lwg3865.cc: New
	test.

2 files changed, 39 insertions, 8 deletions

diff --git a/libstdc++-v3/include/bits/stl_pair.h b/libstdc++-v3/include/bits/stl_pair.h
index 4f5c838..4531741 100644
--- a/libstdc++-v3/include/bits/stl_pair.h
+++ b/libstdc++-v3/include/bits/stl_pair.h
@@ -1000,23 +1000,39 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<typename _T1, typename _T2> pair(_T1, _T2) -> pair<_T1, _T2>;
 #endif
 
-  /// Two pairs of the same type are equal iff their members are equal.
-  template<typename _T1, typename _T2>
+#if __cpp_lib_three_way_comparison && __cpp_lib_concepts
+  // _GLIBCXX_RESOLVE_LIB_DEFECTS
+  // 3865. Sorting a range of pairs
+
+  /// Two pairs are equal iff their members are equal.
+  template<typename _T1, typename _T2, typename _U1, typename _U2>
     inline _GLIBCXX_CONSTEXPR bool
-    operator==(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
+    operator==(const pair<_T1, _T2>& __x, const pair<_U1, _U2>& __y)
     { return __x.first == __y.first && __x.second == __y.second; }
 
-#if __cpp_lib_three_way_comparison && __cpp_lib_concepts
-  template<typename _T1, typename _T2>
-    constexpr common_comparison_category_t<__detail::__synth3way_t<_T1>,
-					   __detail::__synth3way_t<_T2>>
-    operator<=>(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
+  /** Defines a lexicographical order for pairs.
+   *
+   * For two pairs of comparable types, `P` is ordered before `Q` if
+   * `P.first` is less than `Q.first`, or if `P.first` and `Q.first`
+   * are equivalent (neither is less than the other) and `P.second` is
+   * less than `Q.second`.
+  */
+  template<typename _T1, typename _T2, typename _U1, typename _U2>
+    constexpr common_comparison_category_t<__detail::__synth3way_t<_T1, _U1>,
+					   __detail::__synth3way_t<_T2, _U2>>
+    operator<=>(const pair<_T1, _T2>& __x, const pair<_U1, _U2>& __y)
     {
       if (auto __c = __detail::__synth3way(__x.first, __y.first); __c != 0)
 	return __c;
       return __detail::__synth3way(__x.second, __y.second);
     }
 #else
+  /// Two pairs of the same type are equal iff their members are equal.
+  template<typename _T1, typename _T2>
+    inline _GLIBCXX_CONSTEXPR bool
+    operator==(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
+    { return __x.first == __y.first && __x.second == __y.second; }
+
   /** Defines a lexicographical order for pairs.
    *
    * For two pairs of the same type, `P` is ordered before `Q` if
diff --git a/libstdc++-v3/testsuite/20_util/pair/comparison_operators/lwg3865.cc b/libstdc++-v3/testsuite/20_util/pair/comparison_operators/lwg3865.cc
new file mode 100644
index 0000000..2bbd54a
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/pair/comparison_operators/lwg3865.cc
@@ -0,0 +1,15 @@
+// { dg-do run { target c++20 } }
+
+// LWG 3865. Sorting a range of pairs
+
+#include <utility>
+#include <testsuite_hooks.h>
+
+int main()
+{
+  std::pair<int, int> p(1, 2);
+  std::pair<int&, int&> p2(p.first, p.second);
+  VERIFY( p == p2 );
+  VERIFY( p <= p2 );
+  VERIFY( p >= p2 );
+}