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3151. ConvertibleTo rejects conversions from array and function types

Section: 18.4.4 [concept.convertible] Status: Resolved Submitter: Casey Carter Opened: 2018-08-09 Last modified: 2020-11-09

Priority: 3

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Discussion:

In the definition of ConvertibleTo in 18.4.4 [concept.convertible]: 

template<class From, class To>
  concept ConvertibleTo =
    is_convertible_v<From, To> &&
    requires(From (&f)()) {
      static_cast<To>(f());
    };

f is an arbitrary function that returns type From. Since functions cannot return array or function types (9.3.4.6 [dcl.fct] paragraph 11), ConvertibleTo cannot be satisfied when From is an array or function type regardless of the type of To. This is incompatibility with is_convertible_v was not an intentional design feature, so it should be corrected. (Note that any change made here must take care to avoid breaking the ConvertibleTo<T, void> cases.)

[2018-08-20 Priority set to 3 after reflector discussion]

Previous resolution [SUPERSEDED]:

[Drafting Note: I've used declval here, despite that "Concepts mean we never have to use declval again!" because the alternative is less readable:] 

requires(add_rvalue_reference_t<From> (&f)()) {
  static_cast<To>(f());
};

This wording is relative to N4762.

  1. Modify 18.4.4 [concept.convertible] as follows:

    template<class From, class To>
  concept ConvertibleTo =
    is_convertible_v<From, To> &&
    requires(From (&f)()) {
      static_cast<To>(f() declval<From>());
    };

[2020-11-09 Resolved for C++20. Status changed: Tentatively Resolved → Resolved.]

Proposed resolution:

This issue is resolved by the resolution of issue 3194(i).