I'm going to need to revert #2654 and #3255, so here are vNext todos that were mentioned in the latter PR's added comment:

When we can break ABI, we should:

• In <ppltasks.h>, reconsider whether _Task_impl_base::_Wait() should throw invalid_operation; it's questionable whether that's conforming, and if users want to block their UI threads, we should let them.
• Investigate whether we can avoid the ppltasks dependency entirely, making all of these issues irrelevant.