This is initial setup to enabling use of zip() on sentinel based ranges.
Store the range's type in the zip signature, and store the types of
std::begin/std::end for the iterators, rather than assuming that
std::begin and std::end return the same type.
Allow the caller to pick which sequences can terminate the zip iterator.
For compatibility and performance, default to the historical behavior of
examining only the first sequence. The zip iteration terminates when
any of the examined sequences reaches its end, even if other sequences
have not reached their respective end.
The standard type imposes some additional requirements that are not
necessary here, but using this concept allows standard containers to be
classified correctly without specific overrides.
std::ranges::find_if permits use of a sentinel instead of a full
iterator, and supports std::ranges::find as an alternative to certain
simple uses of std::find_if.
Where possible, use the form that takes a range, rather than the form
that takes two iterators.
Add a declared, but not defined, default constructor for
self_return_iterator to satisfy the standard library's concept
`semiregular`, which insists that sentinels be default-constructible,
even for those functions that never need to do so.
Add a defined, but unused, operator++(postfix) for zip_iterator to
satisfy a standard library concept for `forward_iterator`.
Compiler error messages are generally better when reporting a misuse
that fails a requires() versus reporting a misuse that fails a
std::enable_if. In some cases, this also makes the code clearer, and
avoids the need for dummy template parameters as a place to invoke
std::enable_if.
If std::common_type<range::index_type>... finds a common index_type
among all the zipped ranges, pass that common type through as
zip<...>::index_type. Otherwise, set zip<...>::index_type to void.
This allows enumerate(zip(...))) to report a better index_type.
std::find_if needs common iterator traits. Add the relevant type
definitions to zip_iterator.
Also add them to d_range, to avoid errors when a range is zipped.
This is necessarily incomplete since it can only check ranges that have
a compile-time static size. However, it catches some simple mistakes,
and imposes no runtime cost, so it is still useful.
The minimum supported compiler versions now provide a depth-efficient
implementation of std::make_index_sequence, which removes the last
reason to carry a private implementation. In the case of clang, it
appears to have a special compiler intrinsic used to implement its
std::make_index_sequence.
Switch to the compiler-provided version for both gcc and clang.
