This is a pure syntax cleanup, I don't need it for running libc++'s
tests with MSVC's STL.
This is possible because std::layout_meow and layout_wrapping_integral<1729>
aren't dependent types. We only need typename and template when reaching
into a dependent type, like `typename OtherLayout::template mapping<E2>` or
`typename layout_wrapping_integral<Wraps>::template mapping<Extents>`.
Found while running libc++'s test suite with MSVC's STL.
[mdspan.extents.overview]/1.1 mandates that IndexType is a signed or
unsigned integer type, which excludes char.
MSVC's STL enforces the Mandates here, so this PR changes the relevant
occurrences of `char to `signed char`. To make this work, we also need
to add an `operator signed char()` to the test helper type `IntType` so
it remains unambiguously convertible, and then we can remove `operator
char()`.
libc++ should also enforce the Mandates, but this PR doesn't attempt to make
such a change.
This commit implements layout_left in support of C++23 mdspan
(https://wg21.link/p0009). layout_left is a layout mapping policy
whose index mapping corresponds to the memory layout of Fortran arrays.
Thus the left most index has stride-1 access, and the right most index
is associated with the largest stride.
Co-authored-by: Damien L-G <dalg24@gmail.com>
Differential Revision: https://reviews.llvm.org/D153783
