Proposed resolution:

This wording is relative to N5008.

1. Modify [array.zero] as indicated:

   -3- The effect of calling `front()` or `back()` for a zero-sized array is undefined.

The intent of P3471 "Standard library hardening" is clearly to provide hardened preconditions for members of sequence containers, including `std::array`. However, a zero-length `std::array` dodges this hardening by having undefined behavior for `front()` and `back()` explicitly specified in [array.zero] paragraph 3.

Without this paragraph, `front()` and `back()` would be hardened as well, as specified in [sequence.reqmts].