[libcxx-commits] [libcxx] [libc++] Partially implement P2642R6: Padded mdspan layouts (PR #187873)

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eiytoq wrote:

I noticed a gap between the padded layouts and the non-padded layouts.

`layout_{left,right}::mapping::stride` is constrained with `rank() > 0`, but
`layout_{left,right}_padded::mapping::stride` is not.

Should we make them consistent?

CC @frederick-vs-ja 

https://github.com/llvm/llvm-project/pull/187873