<mdspan>: Fix layout_stride::mapping<E>::is_exhaustive()

[JMazurkiewicz]

Currently, our implementation of layout_stride::mapping<E>::is_exhaustive() gives incorrect result for some "unusual" extents. This PR attempts to fix it by adding a new way to compute an answer to the problem described in [mdspan.layout.stride.obs]/5.2 and extra return paths for simple cases:

• When E::rank() == 1, is_exhaustive() returns true if and only if stride(0) == 1.
• When E::rank() == 2, is_exhaustive() returns true if and only if one of the strides is equal to 1 and the other stride is equal to extent corresponding to the previous stride.
• When two or more extents are equal to 0, then is_exhaustive() always returns false.
• When there are no extents equal to 0 or 1 (these are "unusual" extents) this functions uses current method of comparing required_span_size() to fwd-prod-of-extents(rank) (extracted to separate function named _Is_exhaustive_common_case).
• Otherwise, _Is_exhaustive_special_case computes correct answer by finding a permutation of (stride, extent) pairs described in [mdspan.layout.stride.obs]/5.2. If such permutation does not exist this function returns false.
  • Sometimes, when one or more extents are equal to 1, mapping might be exhaustive, but this function (because of the way it is described) should return false. For example:

  extents = [4, 1]
  strides = [1, 5]
  required_span_size = 1 + (4 - 1) * 1 + (1 - 1) * 5 = 4
  fwd-prod-of-extents = 4 * 1 = 4 // equal to required_span_size, `_Is_exhaustive_common_case` would return true
  m(0, 0) = 0
  m(1, 0) = 1
  m(2, 0) = 2
  m(3, 0) = 3

  This mapping is clearly exhaustive according to [mdspan.layout.reqmts]/16, but not per [mdspan.layout.stride.obs]/5.2.

libc++ test fails now due to bogus test - mapping tested here is clearly not exhaustive (at least one of the strides should be equal to 1). This also indicates that their implementation is incorrect.

[StephanTLavavej]

Thanks! 😻 I pushed minor changes, please meow if I messed something up.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for investigating this libcxx skip! 🛠️ 🕵️ 😻