Can anyone explain why the second example below won't compile?

'Test 2' gives "error FS0670: This code is not sufficiently generic. The type variable ^a could not be generalized because it would escape its scope.". I fail to understand this error message. 

// Test 1
type test1<'a> = | A of 'a 
  with
    override t.ToString() = 
      match t with
      | A a -> a.ToString()

// Test 2
type test2<'a> = | A of 'a 
  with
    override t.ToString() = 
      match t with
      | A a -> string a

// Test 3
type test3<'a> = | A of 'a 
  with
    override t.ToString() = 
      match t with
      | A a -> string (a :> obj)
generics f# inline type-inference
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