[ 2017-06-13, Tim Song reopens ]

While it is true that memory_resource* is convertible to polymorphic_allocator, uses-allocator construction still requires allocators, and a memory_resource* isn't an allocator.

To take a concrete example from the current WP, a pmr::vector<std::promise<int>>, as specified, will be attempting to uses-allocator construct a promise<int> with a memory_resource*, but std::promise's allocator-taking constructor expects something that satisfies the allocator requirements, rather than a memory_resource*.