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def-site variance does not prohibit illegal method declarations for interfaces #4287

susisu opened this Issue Jun 30, 2017 · 0 comments


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susisu commented Jun 30, 2017

The def-site variance (I'm not sure this term is correct, found it is used in 945df4d.) can be used for interface declarations, but it allows illegal method declarations.
This makes the type system unsound.

// @flow

class Animal {}
class Cat extends Animal {
  meow() {

interface Foo<+T> {    // declare T as a covariant type parameter
  read(): T;           // this is ok
  write(val: T): void; // this should not be allowed because T is in a contravariant position

const foo : Foo<Cat> = {
  val: new Cat(),
  read(): Cat {
    return this.val;
  write(val: Cat) {
    this.val = val;

(foo: Foo<Animal>); // this "up-casting" is no longer safe
(foo: Foo<Animal>).write(new Animal());; // -> TypeError: is not a function

In this example, the interface Foo<+T> is declared with a method write(val: T): void which is inconsistent with T's covariance.
Nevertheless, Foo<Cat> can still be up-casted to Foo<Animal>, it can cause a runtime error.

@vkurchatkin vkurchatkin added the bug label Jun 30, 2017

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