[libcxx-commits] [PATCH] D107098: [libc++] Implement the underlying mechanism for range adaptors

[Tim Song via Phabricator via libcxx-commits]

tcanens added a comment.

Just a quick note: when P2281R1 <https://wg21.link/P2281R1> talks about perfect forwarding call wrappers, it really means perfect forwarding call wrappers exactly as specified in the standard <http://eel.is/c++draft/func.require#def:perfect_forwarding_call_wrapper>, that is:

> A //perfect forwarding call wrapper// is an argument forwarding call wrapper that forwards its state entities to the underlying call expression. This forwarding step delivers a state entity of type `T` as  `cv T&` when the call is performed on an lvalue of the call wrapper type and as `cv T&&` otherwise, where `cv` represents the cv-qualifiers of the call wrapper and where `cv` shall be neither `volatile` nor `const volatile`.

In particular, invoking an rvalue wrapper must deliver the state entity as an xvalue and cannot fall back to const lvalue the way `__perfect_forward` does (without deducing `this`, I believe that this requires additional deleted `operator()` overloads). This is particularly significant for `split` - see the examples in the paper.


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