* 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
