[Need TypeScript help] Type inference with types.maybe doesn't make property optional

[jamonholmgren]

With this model definition:

const SomeStore = types.model("SomeStore", {
  someData: types.maybe(types.frozen())
});

interface SomeStoreType extends Instance<typeof SomeStore> {}

... someData should be optional when using the inferred type, but, but isn't.

const store: SomeStoreType = {
  // why do we need someData here
};

CodeSandbox

TypeScript Playground of minimal repro

This PR adds a test that demonstrates the issue. If we find a fix, we'll add it to this PR.

[adamkovalsky]

@jamonholmgren you could wrap the Instance type with a type mapper which makes all fields in the model which accept undefined optional, something like this:

type UndefinedIsOptional<Type> =
    & {
        [Prop in keyof Type as (undefined extends Type[Prop] ? Prop : never)]?: Type[Prop];
    }
    & {
        [Prop in keyof Type as (undefined extends Type[Prop] ? never : Prop)]: Type[Prop];
    }

type Model = {
    foo: number | undefined;
    bar: number;
};

const obj: UndefinedIsOptional<Model> = {
    bar: 5
};

[coolsoftwaretyler]

Hey folks, this PR is almost a year old and still just a failing test. I'm gonna go ahead and close it out to clean up our repo.

Happy to review a new PR that builds on this work and resolves it.

Thanks!