libstdc++: Add static assertions to futures and promises [LWG 3458]

LWG recently decided it should be ill-formed to instantiate std::future
and std::shared_future for types that can't be returned from a function.
This adds static assertions to enforce it (std::future already failed,
but this makes the error more understandable).

LWG 3466 extends that to std::promise. The actual constraint is that
t.~T() is well-formed for the primary template, but rejecting arrays and
functions as done for futures matches that condition.

libstdc++-v3/ChangeLog:

	* include/std/future (future, shared_future, promise): Add
	static assertions to the primary template to reject array and
	function types.
	* testsuite/30_threads/future/requirements/lwg3458.cc: New test.
	* testsuite/30_threads/promise/requirements/lwg3466.cc: New test.
	* testsuite/30_threads/shared_future/requirements/lwg3458.cc: New test.

1 files changed, 15 insertions, 0 deletions

diff --git a/libstdc++-v3/include/std/future b/libstdc++-v3/include/std/future
index 6eef6864f4d..bdf4a75d694 100644
--- a/libstdc++-v3/include/std/future
+++ b/libstdc++-v3/include/std/future
@@ -763,6 +763,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<typename _Res>
     class future : public __basic_future<_Res>
     {
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 3458. Is shared_future intended to work with arrays or function types?
+      static_assert(!is_array<_Res>{}, "result type is not an array");
+      static_assert(!is_function<_Res>{}, "result type is not a function");
+
       friend class promise<_Res>;
       template<typename> friend class packaged_task;
       template<typename _Fn, typename... _Args>
@@ -893,6 +898,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<typename _Res>
     class shared_future : public __basic_future<_Res>
     {
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 3458. Is shared_future intended to work with arrays or function types?
+      static_assert(!is_array<_Res>{}, "result type is not an array");
+      static_assert(!is_function<_Res>{}, "result type is not a function");
+
       typedef __basic_future<_Res> _Base_type;
 
     public:
@@ -1045,6 +1055,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<typename _Res>
     class promise
     {
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 3466: Specify the requirements for promise/future/[...] consistently
+      static_assert(!is_array<_Res>{}, "result type is not an array");
+      static_assert(!is_function<_Res>{}, "result type is not a function");
+
       typedef __future_base::_State_base 	_State;
       typedef __future_base::_Result<_Res>	_Res_type;
       typedef __future_base::_Ptr<_Res_type>	_Ptr_type;