# Dubious evidence

[empty-types] [equality]

While the Empty type of the previous counterexample has no values at all, there are types whose emptiness depends on their parameters. The canonical example is the equality type, expressible as a GADT in Haskell or OCaml or as an inductive family in Coq or Agda (among others):

type (_, _) eq =
Refl : ('a, 'a) eq

data Eq a b where
Refl :: Eq a a

Inductive eq {S : Set} : S -> S -> Prop :=
Refl a : eq a a.

data Eq {S : Set} : S -> S -> Set where
refl : (a : S) -> Eq a a


The type (a, b) eq witnesses equality: it is nonempty if a and b are the same type, and empty if they are distinct. Values of type (a, b) eq constitute evidence that a and b are in fact the same, and can be used e.g. to turn an a into a b.

For an arbitrary type a, we have no way of making an (int, a) eq: for all we know a might be string. The only way to construct a value of the eq type is using Refl, which demands the parameters be equal.

In a pure, total language, the existence of an expression of type (a, b) eq is enough to conclude a and b are equal. But in a less pure language, where evaluation might fail or loop, the mere existence of an expression with the right type is not evidence enough: we need to actually run it, to see that it does in fact yield Refl. Several type systems have had soundness bugs where evidence of this sort is trusted without being fully evaluated:

• Null evidence

In languages with null, types like eq contain null, and unlike Refl, null implies nothing about its type parameters. So, before making use of any evidence we need to verify that the evidence isn't null.

The lack of such verification caused a soundness issue in Java and Scala1 (using a type that provides evidence for subtyping, rather than equality):

// Counterexample by Nada Amin and Ross Tate
class Unsound {
static class Constrain<A, B extends A> {}
static class Bind<A> {
<B extends A>
A upcast(Constrain<A,B> constrain, B b) {
return b;
}
}
static <T,U> U coerce(T t) {
Constrain<U,? super T> constrain = null;
Bind<U> bind = new Bind<U>();
return bind.upcast(constrain, t);
}
public static void main(String[] args) {
String zero = Unsound.<Integer,String>coerce(0);
}
}

// Counterexample by Nada Amin and Ross Tate
object unsoundMini {
trait A { type L >: Any}
def upcast(a: A, x: Any): a.L = x
val p: A { type L <: Nothing } = null
def coerce(x: Any): Nothing = upcast(p, x)
coerce("Uh oh!")
}

• Self-justifying evidence

Evidence p : (int, a) eq can be used to construct more evidence that int and a are equal, by seeing that p is Refl, therefore learning that int and a are equal, and using this information to justify Refl : (int, a) eq.

OCaml2 had a soundness issue in which it allowed this sort of reasoning in a recursive definition of p, allowing p to be used as evidence for itself:

(* Counterexample by Stephen Dolan *)
type (_,_) eq = Refl : ('a, 'a) eq
let cast (type a) (type b) (Refl : (a, b) eq) (x : a) = (x : b)

let is_int (type a) =
let rec (p : (int, a) eq) = match p with Refl -> Refl in
p

let bang =
(* segfaults *)
print_string (cast (is_int : (int, string) eq) 42)

• Out-of-order evidence

When nontermination is possible, it is important to ensure that the order in which evidence is used matches the evaluation order. Otherwise, it is possible to write an expression that does not terminate, but make use of it before it runs. Such forward references caused a soundness issue in Scala3:

// Counterexample by Paolo G. Giarrusso
new {
val a: String = (((1: Any): b.A): Nothing): String
def loop(): Nothing = loop()
val b: { type A >: Any <: Nothing } = loop()
}

• Time-travelling evidence

Staged metaprogramming allows a program to manipulate program fragments, gluing together an output program from quoted fragments of code. However, it is unsound to allow evidence computed in the future by the generated output program to justify computations now. This caused a soundness bug in Scala's implementation of staged metaprogramming4 and in BER MetaOCaml5:

// Counterexample by Lionel Parreaux
import scala.quoted.staging._

trait T { type A >: Any <: Nothing }

withQuoteContext { '{ (x: T) => \${ 42: x.A } } }
// crashes with java.lang.ClassCastException

(* Counterexample by Jeremy Yallop *)
type _ t = T : string t

let f : type a. a t option code -> a -> unit code =
fun c x -> .< match .~c with
| None -> ()
| Some T -> .~(print_endline x; .<()>.) >.

let _ = f .< None >. 0

1

Java and Scala’s Type Systems are Unsound (OOPSLA '16) Nada Amin and Ross Tate (2016)