template<ranges::input_range R> requires same_as<R, remove_cvref_t<R>> constexpr range_format format_kind<R> = see below;

A program that instantiates the primary template of format_kind is ill-formed.

For a type R, format_kind<R> is defined as follows:

• If same_as<remove_cvref_t<ranges​::​range_reference_t<R>>, R> is true, format_kind<R> is range_format​::​disabled.

  [Note 1:

  This prevents constraint recursion for ranges whose reference type is the same range type.

  For example, std​::​filesystem​::​path is a range of std​::​filesystem​::​path.

  — end note]

• Otherwise, if the qualified-id R​::​key_type is valid and denotes a type:

  • If the qualified-id R​::​mapped_type is valid and denotes a type, let U be remove_cvref_t<ranges​::​range_reference_t<R>>.

    If either U is a specialization of pair or U is a specialization of tuple and tuple_size_v<U> == 2, format_kind<R> is range_format​::​map.

  • Otherwise, format_kind<R> is range_format​::​set.

• Otherwise, format_kind<R> is range_format​::​sequence.

Remarks: Pursuant to [namespace.std], users may specialize format_kind for cv-unqualified program-defined types that model ranges​::​input_range.

Such specializations shall be usable in constant expressions ([expr.const]) and have type const range_format.