libstdc++: Ensure that _Utf_view is always a view.

Previously, _Utf_view accepted any input_range, including reference-to-array
types like char(&)[2], and stored it as the _M_base member. In such cases,
_Utf_view was not assignable, failing the requirements of view concept.

This patch addresses the issue by adding the ranges::view constraint to the
second template parameter of _Utf_view, and for clarity renaming it from
_Range to _View. The constructor is also adjusted to accept its argument
by value (views must be O(1) move-constructible). This prevents implicitly
generated CTAD from deducing a reference type.

This makes _Utf_view consistent with both other standard views and the
wording from P2728R8: Unicode in the Library, Part 1: UTF Transcoding [1].

The explicit CTAD from viewable_range is not defined for _Utf_view because
it depends on views::all_t, views::ref_view, and views::owning_view,
which are declared in <ranges>. Consequently, users must explicitly cast
the argument to a view or specify it as a template parameter.

[1] https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p2728r8.html

libstdc++-v3/ChangeLog:

	* include/bits/unicode.h (_Utf_view): Rename the template parameter
	from _Range to _View and constrain it with ranges::view.
	(_Utf_view::_Utf_view): Accept by value instead of rvalue reference.
	* include/std/format (__format::__write_padded): Replace _Utf_view
	over const char32_t(&)[1] with span<const char32_t, 1>.
	* testsuite/ext/unicode/view.cc: Add checks if specialization
	of _Utf_view satisfy view. Wrap arrays into std::span before
	constructing _Utf_view.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>

0 files changed, 0 insertions, 0 deletions