Using a union allowed undefined behavior for 8-bit integer INPUT and OUTPUT arguments through a 32-bit integer (and vice versa).
Write throught union::uint8_t[16] and read thoutght union::uint32_t[4]
is a undefined behavior.
C++ standard says (https://timsong-cpp.github.io/cppwp/n4659/class.union#1):
"At most one of the non-static data members of an object of union type
can be active at any time, that is, the value of at most one of the
non-static data members can be stored in a union at any time. [ Note:
One special guarantee is made in order to simplify the use of unions:
If a standard-layout union contains several standard-layout structs
that share a common initial sequence, and if a non-static data member
of an object of this standard-layout union type is active and is one
of the standard-layout structs, it is permitted to inspect the common
initial sequence of any of the standard-layout struct members; see
[class.mem]. — end note ]"
and (https://timsong-cpp.github.io/cppwp/n4659/class.mem#22):
"Two standard-layout unions are layout-compatible if they have the same number of non-static data members and corresponding non-static data members (in any order) have layout-compatible types."
you can't hope that compilers don't seem to do undefined behavior at the moment
Also created espressif/arduino-esp32#8829 PR dependent on changing the current API.