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deduction guides for std::ranges::subrange

Defined in header <ranges> 
template< std::input_or_output_iterator I, std::sentinel_for<I> S >
subrange(I, S) -> subrange<I, S>;
 (1) (since C++20) 
template< std::input_or_output_iterator I, std::sentinel_for<I> S >
subrange(I, S, /*make-unsigned-like-t*/<std::iter_difference_t<I>>) ->
    subrange<I, S, ranges::subrange_kind::sized>;
 (2) (since C++20) 
template< ranges::borrowed_range<R> >
subrange(R&&) ->
    subrange<ranges::iterator_t<R>, ranges::sentinel_t<R>,
             (ranges::sized_range<R> ||
              std::sized_sentinel_for<ranges::sentinel_t<R>,
              ranges::iterator_t<R>>) ?
             ranges::subrange_kind::sized : ranges::subrange_kind::unsized>;
 (3) (since C++20) 
template< ranges::borrowed_range<R> >
subrange(R&&, /*make-unsigned-like-t*/<ranges::range_difference_t<R>>) ->
    subrange<ranges::iterator_t<R>, ranges::sentinel_t<R>,
             ranges::subrange_kind::sized>;
 (4) (since C++20)

These deduction guides are provided for std::ranges::subrange.

1) Deduces the template arguments from the type of iterator and sentinel. The subrange is sized if std::sized_sentinel_for<S, I> is satisfied, as determined by the default template argument.
2) Deduces the template arguments from the type of iterator and sentinel, while the size of range is specified. The subrange is always sized.
3) Deduces the template arguments from the type of range. The subrange is sized if the size can be obtained from the range or its iterator and sentinel.
4) Deduces the template arguments from the type of range, while the size of range is specified. The subrange is always sized.

The exposition-only alias template make-unsigned-like-t maps each integer-like type to its corresponding unsigned version.

Notes

While constructing the subrange object,

  • for (1,2), the behavior is undefined if the iterator-sentinel pair does not denote a valid range,
  • for (2,4), the behavior is undefined if the given size is not equal to the size of the range.

Example

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 3404 C++20 meaningless deduction guides from pair-like types were provided removed

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