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### 2486. `mem_fn()` should be required to use perfect forwarding

Section: 20.14.4 [func.require] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2015-03-27 Last modified: 2017-07-30

Priority: 0

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Discussion:

20.14.4 [func.require]/4 defines "simple call wrapper" and "forwarding call wrapper". Only `mem_fn()` is specified to be a "simple call wrapper", by 20.14.16 [func.memfn]/1: "A simple call wrapper (20.9.1) `fn` such that the expression `fn(t, a2, ..., aN)` is equivalent to `INVOKE(pm, t, a2, ..., aN)` (20.9.2)."

This suggests, but doesn't outright state, that perfect forwarding is involved. It matters for PMFs like `R (T::*)(Arg)` where `Arg` is passed by value — if the `mem_fn()` wrapper's function call operator takes `Arg` by value, an extra copy/move will be observable. We should require perfect forwarding here.

[2015-05, Lenexa]

Move to Immediate.

Proposed resolution:

This wording is relative to N4296.

1. Change 20.14.4 [func.require] as depicted [Editorial remark: This simply adds "A simple call wrapper is a forwarding call wrapper", then moves the sentence. — end of remark]:

   > -4- Every call wrapper (20.9.1) shall be `MoveConstructible`. A simple call wrapper is a call wrapper that is `CopyConstructible` and `CopyAssignable` and whose copy constructor, move constructor, and assignment operator do not throw exceptions. A forwarding call wrapper is a call wrapper that can be called with an arbitrary argument list and delivers the arguments to the wrapped callable object as references. This forwarding step shall ensure that rvalue arguments are delivered as rvalue-references and lvalue arguments are delivered as lvalue-references. A simple call wrapper is a forwarding call wrapper that is `CopyConstructible` and `CopyAssignable` and whose copy constructor, move constructor, and assignment operator do not throw exceptions. [Note: In a typical implementation […] — end note]