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4417. views::indices is underconstrained

Section: 25.6.4.1 [range.iota.overview] Status: Tentatively NAD Submitter: Hewill Kang Opened: 2025-10-15 Last modified: 2026-01-16

Priority: Not Prioritized

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Discussion:

Whether an integer-class type satisfies weakly_incrementable is unspecified according to 25.6.4.2 [range.iota.view]. For example, the library may provide a member type alias difference_type for the integer-class type to make it weakly_incrementable, or not.

If the latter, then views::iota(integer-class-type(0)) is always ill-formed because iota_view<W, Bound> requires W to be weakly_incrementable.

However, unlike views::iota, views::indices unconditionally accepts integer-class types, which will lead to a hard error in the function body if the integer-class type is not weakly_incrementable, which is true for both libstdc++ and MSVC-STL.

[2026-01-16; Reflector poll. Status → Tentatively NAD.]

"This is implied by the expression-equivalence."

Proposed resolution:

This wording is relative to N5014.

  1. Modify 25.6.4.1 [range.iota.overview] as indicated:

    -4- The name views::indices denotes a customization point object (16.3.3.3.5 [customization.point.object]). Given subexpression E, let T be remove_cvref_t<decltype((E))>. views::indices(E) is expression-equivalent to views::iota(T(0), E) if is-integer-like<T> is true and T models weakly_incrementable, and ill-formed otherwise.