libstdc++: Implement C++23 P1951R1 (Default Args for pair's Forwarding Ctor) [PR105505]

This was approved for C++23 at he June 2021 virtual meeting.

This allows more efficient construction of std::pair members when {} is
used as a constructor argument. The perfect forwarding constructor can
be used, so that the member variables are constructed from forwarded
arguments instead of being copy constructed from temporaries.

libstdc++-v3/ChangeLog:

	PR libstdc++/105505
	* include/bits/stl_pair.h (pair::pair(U1&&, U2&&)) [C++23]: Add
	default template arguments, as per P1951R1.
	* testsuite/20_util/pair/cons/default_tmpl_args.cc: New test.

2 files changed, 56 insertions, 0 deletions

diff --git a/libstdc++-v3/include/bits/stl_pair.h b/libstdc++-v3/include/bits/stl_pair.h
index ce8826a..52f532f 100644
--- a/libstdc++-v3/include/bits/stl_pair.h
+++ b/libstdc++-v3/include/bits/stl_pair.h
@@ -308,7 +308,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       { }
 
       /// Constructor accepting two values of arbitrary types
+#if __cplusplus > 202002L
+      template<typename _U1 = _T1, typename _U2 = _T2>
+#else
       template<typename _U1, typename _U2>
+#endif
 	requires (_S_constructible<_U1, _U2>()) && (!_S_dangles<_U1, _U2>())
 	constexpr explicit(!_S_convertible<_U1, _U2>())
 	pair(_U1&& __x, _U2&& __y)
@@ -316,7 +320,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	: first(std::forward<_U1>(__x)), second(std::forward<_U2>(__y))
 	{ }
 
+#if __cplusplus > 202002L
+      template<typename _U1 = _T1, typename _U2 = _T2>
+#else
       template<typename _U1, typename _U2>
+#endif
 	requires (_S_constructible<_U1, _U2>()) && (_S_dangles<_U1, _U2>())
 	constexpr explicit(!_S_convertible<_U1, _U2>())
 	pair(_U1&&, _U2&&) = delete;
diff --git a/libstdc++-v3/testsuite/20_util/pair/cons/default_tmpl_args.cc b/libstdc++-v3/testsuite/20_util/pair/cons/default_tmpl_args.cc
new file mode 100644
index 0000000..5960bf7
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/pair/cons/default_tmpl_args.cc
@@ -0,0 +1,48 @@
+// { dg-do compile { target c++23 } }
+
+// P1951R1 Default Arguments for pair's Forwarding Constructor
+
+#include <utility>
+#include <vector>
+#include <string>
+#include <testsuite_hooks.h>
+
+void
+test_p1951r1_example()
+{
+  std::pair<std::string, std::vector<std::string>> p("hello", {});
+}
+
+struct Counter
+{
+  constexpr Counter() = default;
+  constexpr Counter(int) { }
+  constexpr Counter(const Counter& c) : copies(c.copies + 1), moves(c.moves) { }
+  constexpr Counter(Counter&& c) : copies(c.copies), moves(c.moves+1) { }
+  int copies = 0;
+  int moves = 0;
+};
+
+constexpr bool
+test_count_copies()
+{
+  std::pair<Counter, Counter> p1(1, {});
+  VERIFY( p1.first.copies == 0 && p1.second.copies == 0 );
+  VERIFY( p1.first.moves == 0 && p1.second.moves == 1 );
+
+  std::pair<Counter, Counter> p2({}, 1);
+  VERIFY( p2.first.copies == 0 && p2.second.copies == 0 );
+  VERIFY( p2.first.moves == 1 && p2.second.moves == 0 );
+
+  std::pair<Counter, Counter> p3({}, {});
+  VERIFY( p3.first.copies == 0 && p3.second.copies == 0 );
+  VERIFY( p3.first.moves == 1 && p3.second.moves == 1 );
+
+  return true;
+}
+
+int main()
+{
+  test_p1951r1_example();
+  static_assert( test_count_copies() );
+}