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Structural typing undermining type safety #5933
Description
class Dog {furColor: string}
class Cat {}
function walk(d: Dog): void {}
function pet(c: Cat): void {}
const d: Dog = {furColor: "brown"};
const c: Cat = {};
walk(c); // error, good!
pet(d); // no error, bad!Ryan and I have had extensive conversations about this problem. In my opinion this should clearly fail since both Dog and Cat are classes. Type safety should ensure that when we demand a class type the thing passed in should be in the inheritance chain. Structural typing shouldn't apply to classes.
There's an argument to be made that this case also shouldn't be supported for interfaces, which I'm also sympathetic to. This similar example proves the problem:
interface Dog {furColor: string}
interface Cat {}
function walk(d: Dog): void {}
function pet(c: Cat): void {}
const d: Dog = {furColor: "brown"};
const c: Cat = {};
walk(c); // error, good!
pet(d); // no error, bad!However, if we mark furColor as optional, I think there's a totally valid argument to be made that it should compile and the desired behavior is met, e.g.
interface Dog {furColor?: string}
interface Cat {}
function walk(d: Dog): void {}
function pet(c: Cat): void {}
const d: Dog = {furColor: "brown"};
const c: Cat = {};
walk(c); // no error, good!
pet(d); // no error, good!
Activity
zpdDG4gta8XKpMCd commentedon Dec 5, 2015
so what is the take out of what you said? what's the action item?
Arnavion commentedon Dec 6, 2015
A similar thing is the ability to assign object literals to class types as long as they have the same members, but currently
constructoris not such a required member. So this issue is a prerequisite for #3841mhegazy commentedon Dec 8, 2015
This is an issue that we have talked about on and off over the past years. for this to proceed we will need a proposal for how this fits in the current system, what are the proposed new rules for comparing class types, what impact of a change does this have on existing code (we can help out with part by running the ts internal snapshot of real world code bases).
RyanCavanaugh commentedon Jul 17, 2020
I think some day we might revisit class nominality, but we haven't seen any proposals we really like yet