Dealing with ABI incompatibilities between different implementations
is outside the scope of the standard.

In any case, just checking if the result of std::layout_of (in your
case) matches expectations doesn't seem sufficient. You can implement
std::span using a pair of pointers or using a pointer and a size for
example, and both would likely have a layout of {8, 8, 8, 8} but this
doesn't tell you anything about padding bytes yet, or which
implementation was actually used.