libstdc++: Adjust span/mdspan CTAD for P2781R9.

A usecase for P2781R9 is more ergonomic creation of span and mdspan with
mixed static and dynamic extents, e.g.:

    span(ptr, cw<3>)
    extents(cw<3>, 5, cw<7>)
    mdspan(ptr, cw<3>, 5, cw<7>)

should be deduced as:
    span<..., 3>
    extents<..., 3, dyn, 7>
    mdspan<..., extents<..., 3, dyn, 7>>

The change required is to strip cv-qualifiers and references from
`_Tp::value`, because of:

    template<_CwFixedValue _X, typename>
      struct constant_wrapper : _CwOperators
      {
	static constexpr const auto& value = _X._M_data;

libstdc++-v3/ChangeLog:

	* include/std/span (__integral_constant_like): Allow the member
	`value` of a constant wrapping type to be a const reference of
	an integer.
	* testsuite/23_containers/mdspan/extents/misc.cc: Add test for
	cw and constant_wrapper.
	* testsuite/23_containers/mdspan/mdspan.cc: Ditto.
	* testsuite/23_containers/span/deduction.cc: Ditto.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>

0 files changed, 0 insertions, 0 deletions