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Assignability issue with function types and and subclass argument #5673
Description
Hi
I stumbled on a strange behaviour which in my understanding is a bug (in Node with Typescript 1.6 and reproduced in the online Playground)
B is a subclass of A
It is possible to assign a variable F of type "function with A argument" with a value "function with a B argument".
This means that when calling F with a value of type A, the concrete function called will use some specific properties of B which are missing, leading to a runtime error
Olivier
class Mammal {
}
class Whale extends Mammal {
swim(duration:number) { /* some code */ }
}
function swim10(w:Whale) { w.swim(10); }
type mammalFunc = (m:Mammal) => void;
function each(m:Mammal[], f:(Mammal) => void) { m.forEach(f); }
var f1:mammalFunc = swim10;
// Problem : this should not be possible
var m1 = new Mammal();
f1(m1);
// this compiles, but will fail at runtime because m1 has no swim() method
each([m1], swim10);
// this compiles, but will also fail at runtime
Activity
ahejlsberg commentedon Nov 15, 2015
This is by design. See #222 and the explanation here. It is certainly possible to design rules that would disallow the example above (e.g. we could have strict contra-variance for parameters), but it would have far ranging undesired effects. For example, with strict parameter contra-variance, arrays would become invariant and a
Whale[]wouldn't be assignable to aMammal[]. That runs counter to everyone's expectations. So, in short, by fixing this we'd just trade one problem for another.olivr70 commentedon Nov 17, 2015
Thanks for the explanation. I missed it in the doc.