3247. ranges::iter_move should perform ADL-only lookup of iter_move

Section: 24.3.3.1 [iterator.cust.move] Status: C++20 Submitter: Casey Carter Opened: 2019-07-29 Last modified: 2021-02-25

Priority: 2

View all other issues in [iterator.cust.move].

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Discussion:

The specification of the ranges::iter_move customization point in [iterator.cust.move] doesn't include a deleted poison pill overload. There is no std::iter_move to avoid, so such a poison pill is not needed. This is fine, except that it suggests that unqualified lookup for iter_move in the iter_move(E) expression in paragraph 1.1 should find declarations of iter_move in the global namespace via normal unqualified lookup, when the design intent is that only ADL be used to find overloads of iter_move.

Absent a more idiomatic wording to specify an ADL-only lookup, we can correct the problem by specifying the lookup in paragraph 1.1 is performed in a context that includes the declaration "void iter_move();" which has the desired effect.

[2020-02 Status to Immediate on Thursday morning in Prague.]

Proposed resolution:

This wording is relative to N4820.

[Drafting Note: There's a drive-by fix here to change "valid" — which suggests runtime behavior which cannot be validated at compile time — to "well-formed".]

1. Modify 24.3.3.1 [iterator.cust.move] as indicated:

   -1- The name ranges::iter_move denotes a customization point object (16.3.3.3.5 [customization.point.object]). The expression ranges::iter_move(E) for some subexpression E is expression-equivalent to:

   1. (1.1) — iter_move(E), if that expression is validwell-formed when treated as an unevaluated operand, with overload resolution performed in a context that does not include a declaration of ranges::iter_move. but does include the declaration:

         void iter_move();
      

   2. […]