PR libstdc++/80493 fix invalid exception specification

	PR libstdc++/80493
	* include/experimental/optional (optional::swap): Fix exception
	specification.

From-SVN: r247093

libstdc++-v3/ChangeLog

+2017-04-24  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR libstdc++/80493
+	* include/experimental/optional (optional::swap): Fix exception
+	specification.
+
 2017-04-21  Jonathan Wakely  <jwakely@redhat.com>
 
 	PR libstdc++/80316

libstdc++-v3/include/experimental/optional

@@ -690,7 +690,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       void
       swap(optional& __other)
       noexcept(is_nothrow_move_constructible<_Tp>()
-               && noexcept(swap(declval<_Tp&>(), declval<_Tp&>())))
+               && __is_nothrow_swappable<_Tp>::value)
       {
         using std::swap;