Self Referencing Generics only work when optional

[NielsCo]

The following TypeSpec Code:

model MyModel<T extends MyModel<T>> {
    // otherProperties
    link: T;
}

model ExtendsMyModel extends MyModel<ExtendsMyModel> {
  // some other properties
}

Throws the following Error: error missing-property Property 'link' is missing on type 'ExtendsMyModel' but required in 'MyModel<T>' Playground Link

Making the link-property optional makes it work though:

model MyOptionalModel<T extends MyOptionalModel<T>> {
    // otherProperties
    link?: T;
}

model ExtendsMyOptionalModel extends MyOptionalModel<ExtendsMyOptionalModel> {
  // some other properties
}

Playground Link

Maybe this has to do with the warning in the interface docs

[timotheeguerin]

interesting, from looking at the error this seems to be due to the override check for model extends that check properties in derived models are compatible with the base model

[markcowl]

@NielsCo Can you elaborate on the scenario where you would use this sort of API definition?

[NielsCo]

Sure. If I someone wanted to model a tree data-structure they could create a model like this. With the "link" property being the parent-node. And the need for generics because there are multiple node-types. Which obviously only is a good idea if the tree structure is and (will forever be) small enough to be transmitted via an endpoint.

I probably should have elaborated that for me the issue is not a blocker, as I was just trying out TypeSpec and noticed that this feature I know from other languages didn't work as I expected it to. Especially the part that the property being optional did not cause an error, looked like an interesting issue to me

[markcowl]

Thanks, that helps. Definitely an issue we want to fix, but this helps us weigh the priority