promote - determine promoted type of two types
type1.promote(type2)
The promote() method determines the common
type that both type1 and type2 can be safely promoted
to for arithmetic or comparison operations.
A Type value representing the promoted type,
or nil if no promotion is possible.
ArgCheck if exactly one argument is not
provided.
TypeCheck if the arguments are
not types.
typeof(1).promote(typeof(1.5))
Float