To precise, for a caller, it is easier to see what is the expecting type without anchor type, otherwise he has to digg into the interface of HTTP_AUTHORIZATION to see what is the type of `login'

And if we would really really want to use anchor type, I would do
make_basic_auth (u: attached like login; p: attached like password)

but I still think it is easier to read
make_basic_auth (u: READABLE_STRING_32; p: READABLE_STRING_32)


Especially if one day we use IMMUTABLE_STRING_32  for the type of `login' and `password'
the `attached like login' would be annoying for user since there is no implicit conversion for instance from STRING_8 to IMMUTABLE_STRING_32