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### 3420. `cpp17-iterator` should check that the type looks like an iterator first

Section: 25.3.2.3 [iterator.traits] Status: WP Submitter: Tim Song Opened: 2020-02-29 Last modified: 2020-11-09

Priority: 0

View all other issues in [iterator.traits].

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Discussion:

It is common in pre-C++20 code to rely on SFINAE-friendly `iterator_traits` to rule out non-iterators in template constraints (`std::filesystem::path` is 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 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<Foo>);

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 25.3.2.3 [iterator.traits] as indicated:

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

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

   > >     […]