LWG 2921 remove packaged_task constructors taking allocators

	* doc/xml/manual/evolution.xml: Document LWG DR 2921 change.
	* doc/xml/manual/intro.xml: Likewise.
	* include/std/future (__create_task_state): Add default arguments
	to make providing an allocator optional.
	(packaged_task::packaged_task(F&&)): Call __create_task_state directly
	instead of delegating to another constructor.
	(packaged_task::packaged_task(allocator_arg_t, const A&, ...)): Do not
	define allocator-extended constructors for C++17 and later.
	* testsuite/30_threads/packaged_task/cons/alloc.cc: Only run test for
	C++11 and C++14.
	* testsuite/30_threads/packaged_task/cons/alloc2.cc: Likewise.
	* testsuite/30_threads/packaged_task/cons/alloc_min.cc: Likewise.
	* testsuite/30_threads/packaged_task/uses_allocator.cc: Likewise.

From-SVN: r271582

1 files changed, 28 insertions, 21 deletions

diff --git a/libstdc++-v3/include/std/future b/libstdc++-v3/include/std/future
index 9671100..6c27669 100644
--- a/libstdc++-v3/include/std/future
+++ b/libstdc++-v3/include/std/future
@@ -1447,9 +1447,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       } _M_impl;
     };
 
-  template<typename _Signature, typename _Fn, typename _Alloc>
+  template<typename _Signature, typename _Fn,
+	   typename _Alloc = std::allocator<int>>
     static shared_ptr<__future_base::_Task_state_base<_Signature>>
-    __create_task_state(_Fn&& __fn, const _Alloc& __a)
+    __create_task_state(_Fn&& __fn, const _Alloc& __a = _Alloc())
     {
       typedef typename decay<_Fn>::type _Fn2;
       typedef __future_base::_Task_state<_Fn2, _Alloc, _Signature> _State;
@@ -1481,28 +1482,40 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       // Construction and destruction
       packaged_task() noexcept { }
 
-      // _GLIBCXX_RESOLVE_LIB_DEFECTS
-      // 2095.  missing constructors needed for uses-allocator construction
-      template<typename _Allocator>
-	packaged_task(allocator_arg_t, const _Allocator& __a) noexcept
-	{ }
-
       template<typename _Fn, typename = __not_same<_Fn>>
 	explicit
 	packaged_task(_Fn&& __fn)
-	: packaged_task(allocator_arg, std::allocator<int>(),
-			std::forward<_Fn>(__fn))
+	: _M_state(
+	    __create_task_state<_Res(_ArgTypes...)>(std::forward<_Fn>(__fn)))
 	{ }
 
+#if __cplusplus < 201703L
       // _GLIBCXX_RESOLVE_LIB_DEFECTS
-      // 2097.  packaged_task constructors should be constrained
+      // 2097. packaged_task constructors should be constrained
       // 2407. [this constructor should not be] explicit
+      // 2921. packaged_task and type-erased allocators
       template<typename _Fn, typename _Alloc, typename = __not_same<_Fn>>
 	packaged_task(allocator_arg_t, const _Alloc& __a, _Fn&& __fn)
 	: _M_state(__create_task_state<_Res(_ArgTypes...)>(
 		   std::forward<_Fn>(__fn), __a))
 	{ }
 
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 2095.  missing constructors needed for uses-allocator construction
+      template<typename _Allocator>
+	packaged_task(allocator_arg_t, const _Allocator& __a) noexcept
+	{ }
+
+      template<typename _Allocator>
+	packaged_task(allocator_arg_t, const _Allocator&,
+		      const packaged_task&) = delete;
+
+      template<typename _Allocator>
+	packaged_task(allocator_arg_t, const _Allocator&,
+		      packaged_task&& __other) noexcept
+	{ this->swap(__other); }
+#endif
+
       ~packaged_task()
       {
         if (static_cast<bool>(_M_state) && !_M_state.unique())
@@ -1513,19 +1526,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       packaged_task(const packaged_task&) = delete;
       packaged_task& operator=(const packaged_task&) = delete;
 
-      template<typename _Allocator>
-	packaged_task(allocator_arg_t, const _Allocator&,
-		      const packaged_task&) = delete;
-
       // Move support
       packaged_task(packaged_task&& __other) noexcept
       { this->swap(__other); }
 
-      template<typename _Allocator>
-	packaged_task(allocator_arg_t, const _Allocator&,
-		      packaged_task&& __other) noexcept
-	{ this->swap(__other); }
-
       packaged_task& operator=(packaged_task&& __other) noexcept
       {
 	packaged_task(std::move(__other)).swap(*this);
@@ -1577,10 +1581,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	 packaged_task<_Res(_ArgTypes...)>& __y) noexcept
     { __x.swap(__y); }
 
+#if __cplusplus < 201703L
+  // _GLIBCXX_RESOLVE_LIB_DEFECTS
+  // 2976. Dangling uses_allocator specialization for packaged_task
   template<typename _Res, typename _Alloc>
     struct uses_allocator<packaged_task<_Res>, _Alloc>
     : public true_type { };
-
+#endif
 
   // Shared state created by std::async().
   // Holds a deferred function and storage for its result.