typescript-unsound-generics.ts

gistfile1.ts

class Box<T> { constructor(public value: T) {} }

class A {}
class B { foo() {} }

var bb : Box<B> = new Box<B>(null);
// This typechecks (it shouldn't; T is used both in a co- and contra-variant position, so it's invariant)...
var ba : Box<A> = xb;

ba.value = new A();
// ... which causes an error at runtime, here
bb.value.foo();