Add noexcept to std::shared_future copy operations (LWG DR 2799)

	* include/std/future (shared_future): Add noexcept to copy constructor
	and copy-assignment operator (LWG 2799).

From-SVN: r254821

libstdc++-v3/ChangeLog

+2017-11-16  Jonathan Wakely  <jwakely@redhat.com>
+
+	* include/std/future (shared_future): Add noexcept to copy constructor
+	and copy-assignment operator (LWG 2799).
+
 2017-11-15  Jason Merrill  <jason@redhat.com>
 
 	* include/std/numeric (gcd): Fix typo.

libstdc++-v3/include/std/future

@@ -896,7 +896,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       constexpr shared_future() noexcept : _Base_type() { }
 
       /// Copy constructor
-      shared_future(const shared_future& __sf) : _Base_type(__sf) { }
+      shared_future(const shared_future& __sf) noexcept : _Base_type(__sf) { }
 
       /// Construct from a future rvalue
       shared_future(future<_Res>&& __uf) noexcept
@@ -908,7 +908,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       : _Base_type(std::move(__sf))
       { }
 
-      shared_future& operator=(const shared_future& __sf)
+      shared_future& operator=(const shared_future& __sf) noexcept
       {
         shared_future(__sf)._M_swap(*this);
         return *this;