libstdc++: Add conditional noexcept to std::exchange

This is not required by the standard, but seems useful.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	* include/std/utility (exchange): Add noexcept-specifier.
	* testsuite/20_util/exchange/noexcept.cc: New test.

2 files changed, 41 insertions, 0 deletions

diff --git a/libstdc++-v3/include/std/utility b/libstdc++-v3/include/std/utility
index c2697f8..69d274f 100644
--- a/libstdc++-v3/include/std/utility
+++ b/libstdc++-v3/include/std/utility
@@ -91,6 +91,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     _GLIBCXX20_CONSTEXPR
     inline _Tp
     exchange(_Tp& __obj, _Up&& __new_val)
+    noexcept(__and_<is_nothrow_move_constructible<_Tp>,
+		    is_nothrow_assignable<_Tp&, _Up>>::value)
     { return std::__exchange(__obj, std::forward<_Up>(__new_val)); }
 
 #if __cplusplus >= 201703L
diff --git a/libstdc++-v3/testsuite/20_util/exchange/noexcept.cc b/libstdc++-v3/testsuite/20_util/exchange/noexcept.cc
new file mode 100644
index 0000000..6363a4d
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/exchange/noexcept.cc
@@ -0,0 +1,39 @@
+// { dg-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+#include <utility>
+
+// This is a GCC extension. std::exchange is not required to be noexcept.
+
+static_assert( noexcept( std::exchange(std::declval<int&>(), 1) ) );
+
+struct X
+{
+  X(const X&);
+  X(X&&) noexcept;
+  X& operator=(const X&);
+  X& operator=(X&&) noexcept;
+  X& operator=(int);
+};
+
+extern X x, x2;
+static_assert( noexcept( std::exchange(x, std::move(x2)) ) );
+static_assert( ! noexcept( std::exchange(x, 1) ) );
+
+struct Y
+{
+  Y(Y&&) noexcept;
+  Y& operator=(Y&&);
+};
+
+extern Y y, y2;
+static_assert( ! noexcept( std::exchange(y, std::move(y2)) ) );
+
+struct Z
+{
+  Z(Z&&)noexcept;
+  Z& operator=(Z&&) ;
+};
+
+extern Z z, z2;
+static_assert( ! noexcept( std::exchange(z, std::move(z2)) ) );