std::span<T,Extent>::span

1) Constructs an empty span whose data() == nullptr and size() == 0.

• This overload participates in overload resolution only if extent == 0 || extent == std::dynamic_extent.

2) Constructs a span that is a view over the range [first, first + count); the resulting span has data() == std::to_address(first) and size() == count.

• The behavior is undefined if [first, first + count) is not a valid range, if It does not actually model contiguous_iterator, or if extent != std::dynamic_extent && count != extent.
• This overload participates in overload resolution only if

• It satisfies contiguous_iterator
• the conversion from std::iter_reference_t<It> to element_type is at most a qualification conversion.

3) Constructs a span that is a view over the range [first, last); the resulting span has data() == std::to_address(first) and size() == last-first.

• The behavior is undefined if [first, last) is not a valid range, if It does not actually model contiguous_iterator, if End does not actually model sized_sentinel_for for It, or if extent != std::dynamic_extent && last-first != extent.
• This overload participates in overload resolution only if

• It satisfies contiguous_iterator,
• End satisfies sized_sentinel_for for It,
• the conversion from std::iter_reference_t<It> to element_type is at most a qualification conversion, and
• std::is_convertible_v<End, std::size_t> is false.

4-6) Constructs a span that is a view over the array arr; the resulting span has size() == N and data() == std::data(arr).

• These overloads participate in overload resolution only if extent == std::dynamic_extent || N == extent is true and the conversion from std::remove_pointer_t<decltype(data(arr))> to element_type is at most a qualification conversion. These constructor templates are never used for class template argument deduction.

7) Constructs a span that is a view over the range range; the resulting span has size() == std::ranges::size(range) and data() == std::ranges::data(range).

• The behavior is undefined if R does not actually model contiguous_range and sized_range or if R does not model borrowed_range while element_type is non-const or both extent != dynamic_extent and std::ranges::size(range) != extent are true.
• This overload participates in overload resolution only if

• R satisfies contiguous_range and sized_range,
• either R satisfies borrowed_range or std::is_const_v<element_type> is true
• std::remove_cvref_t<R> is not a specialization of std::span,
• std::remove_cvref_t<R> is not a specialization of std::array
• std::is_array_v<std::remove_cvref_t<R>> is false, and
• the conversion from std::ranges::range_reference_t<R> to element_type is at most a qualification conversion.

8) Converting constructor from another span source; the resulting span has size() == source.size() and data() == source.data().

• The behavior is undefined if both extent != dynamic_extent and source.size() != extent are true.
• This overload participates in overload resolution only if at least one of extent == std::dynamic_extent, N == std::dynamic_extent and N == extent is true and the conversion from U to element_type is at most a qualification conversion.

9) Defaulted copy constructor copies the size and data pointer; the resulting span has size() == other.size() and data() == other.data().

2) Throws nothing.

3) Throws what and when last - first throws.

7) Throws what and when std::ranges::size(r) and std::ranges::data(r) throw.