Implicit conversion for erased unions

Test.fs

type U2<'a,'b> =
    | Case1 of 'a
    | Case2 of 'b
    static member op_Implicit(x:'a) = U2.Case1 x
    static member op_Implicit(x:'b) = U2.Case2 x


let inline (!|) (x:^a) : ^b = ((^a or ^b) : (static member op_Implicit : ^a -> ^b) x)

let foo (arg: U2<string, int>) = ()

foo(!|"hello")
foo(!|5)
//foo(!|5.) // Doesn't compile