multi bytes character - how to make it defined behavior?

static_assert('×' == 50071);
GCC -  warning multi byte
CLANG - error character too large
I think instead of "multi bytes" we need "multi characters" - not bytes.
We decode utf8 then we have the character to decide if it is multi char or not.
decoding '×' would consume bytes 195 and 151 the result is the decoded Unicode value of 215.
It is not multi byte : 256*195 + 151 = 50071
O the other hand 'ab' is "multi character" resulting
256 * 'a' + 'b' = 256*97+98= 24930
One consequence is that
'ab' == '𤤰'
But I don't think this is a problem. At least everything is defined.