test.fsx

type Foo<'a> = Foo of string 

let get (Foo key : Foo<'a>) : 'a option =
  None

let set (Foo key : Foo<'a>) (value : 'a) =
  ()


let f = Foo "test"

set f 3

let x = get f
/// ^ Value restriction. The value 'x' has been inferred to have generic type
///   val x : '_a option