F# Cast as Prefix Operator

Question

In F#, many infix operators can be made prefix operators:

a + b
(+) 1 a b

However, this doesn't seem to be allowed for the upcast (or downcast) operators.

x is a UserPrincipal , which inherits from Principal

x :> Principal
(:>) x Principal

Why?

Answer 1

Probably because unlike other operators, the cast operators don't take two expressions as arguments; they take an expression and a type. So (:>) x would have to be "a function that takes a type", which is not a thing that exists in F#.