[Bug 271931] The C++20 feature 'std::ranges::copy' is missing

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=271931

--- Comment #3 from Mark Millard <marklmi26-fbsd@yahoo.com> ---
Trying for some slight useful information this time:

When I look in /usr/include/c++/v1/algorithm I see:

. . .
#include <__algorithm/ranges_copy.h>
. . .

Looking there:

. . .
#if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)

_LIBCPP_BEGIN_NAMESPACE_STD

namespace ranges {

template <class _InIter, class _OutIter>
using copy_result = in_out_result<_InIter, _OutIter>;

namespace __copy {
. . .
} // namespace __copy

inline namespace __cpo {
  inline constexpr auto copy = __copy::__fn{};
} // namespace __cpo
} // namespace ranges

_LIBCPP_END_NAMESPACE_STD

#endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
. . .

Thus:

# c++ -std=c++2b -pedantic -Wall -Wextra -fexperimental-library ranges_copy.cpp
# ./a.out
destination contains: 0 1 2 3 4 5 6 7 8 9 
odd numbers in destination are: 1 3 5 7 9 

from /usr/include/c++/v1/__config having:

#  if __has_feature(experimental_library)
#    ifndef _LIBCPP_ENABLE_EXPERIMENTAL
#      define _LIBCPP_ENABLE_EXPERIMENTAL
#    endif
#  endif

// Incomplete features get their own specific disabling flags. This makes it
// easier to grep for target specific flags once the feature is complete.
#  if !defined(_LIBCPP_ENABLE_EXPERIMENTAL) &&
!defined(_LIBCPP_BUILDING_LIBRARY)
#    define _LIBCPP_HAS_NO_INCOMPLETE_FORMAT
#    define _LIBCPP_HAS_NO_INCOMPLETE_RANGES
#  endif

15.0.7 likely requires -fexperimental-library for many things
to be enabled because of the overall thing being incomplete or
known to be partially broken, such as C++20 ranges overall
having some parts not sufficiently handled (apparently).

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