<future>: Make packaged_task accept move-only functors by frederick-vs-ja · Pull Request #4946 · microsoft/STL (original) (raw)

Fixes #321.

The issue was considered ABI-breaking, but I think it can be resolved in an ABI-preserving way like #2568. This PR adds internal constructors to function to accept non-copy-constructible functors and add static_assert to keep the copyability checking for standard constructors. Valid user codes won't be able to call these internal constructors.

Also implements the previously missing Mandates in [futures.task.members]/3, and switches to use move construction in reset per [futures.task.members]/26.

Notes:

• The internal constructors are made constrained templates to avoid affecting overload resolution unexpectedly, see LLVM-103409.
• The involved allocator-extended constructors were removed in C++17 by WG21-P0302R1 and LWG-2921, so this PR cites WG21-N4140 in the C++17-removed constructors.
• The fix should work for most move-only types. Although there can be classes whose copy constructor are only invalid in instantation. In vNext we should completely get rid of function.
• If the function<R(Args...)> is a program-defined specialization, this approach doesn't work. I don't think any user should specialize std::function, but this is allowed by the standard.
• Three _Packaged_state specializations are merged into one. I believe this can ease maintenance.