Issue 3420: cpp17-iterator should check that the type looks like an iterator first (original) (raw)

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3420. _cpp17-iterator_ should check that the type looks like an iterator first

Section: 24.3.2.3 [iterator.traits] Status: C++23 Submitter: Tim Song Opened: 2020-02-29 Last modified: 2023-11-22

Priority: 0

View all other issues in [iterator.traits].

View all issues with C++23 status.

Discussion:

It is common in pre-C++20 code to rely on SFINAE-friendly iterator_traitsto rule out non-iterators in template constraints (std::filesystem::pathis one example in the standard library).

C++20 changed iterator_traits to automatically detect its members in some cases, and this detection can cause constraint recursion. LWG 3244(i)tries to fix this for path by short-circuiting the check when the source type is path itself, but this isn't sufficient:

struct Foo { Foo(const std::filesystem::path&); };

static_assert(std::copyable);

Here the copyability determination will ask whether a path can be constructed from a Foo, which asks whether Foo is an iterator, which checks whether Foo is copyable […].

To reduce the risk of constraint recursion, we should change _cpp17-iterator_so that it does not ask about copyability until the type is known to resemble an iterator.

[2020-04-04 Issue Prioritization]

Status set to Tentatively Ready after six positive votes on the reflector.

[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4861.

1. Modify 24.3.2.3 [iterator.traits] as indicated:

   -2- The definitions in this subclause make use of the following exposition-only concepts:

   template
   concept cpp17-iterator =
   copyable && requires(I i) {
   { *i } -> can-reference;
   { ++i } -> same_as<I&>;
   { *i++ } -> can-reference;
   } && copyable;

   […]