When two pointers are subtracted, both shall point to elements of the
same array object, or one past the last element of the array object;
the result is the difference of the subscripts of the two array elements.

The size of the result is implementation-defined, and its type
(a signed integer type) is ptrdiff_t defined in the <stddef.h> header.
If the result is not representable in an object of that type,
the behavior is undefined.

See C Standard, §6.5.6 [ISO/IEC 9899:2011]

Fixes #7216