Optional default gets a Typescript complaint

[bryanstearns]

Bug report

When declaring an optional default for a model with entirely defaultable properties, I get a typescript error that I haven't been able to figure out.

• [x] I've checked documentation and searched for existing issues
• [x] I've made sure my project is based on the latest MST version
• [x] Fork this code sandbox or another minimal reproduction.

Sandbox link or minimal reproduction code https://codesandbox.io/s/optionaldefaultproblem-qx08w

import { types } from "mobx-state-tree";

export const ChildModel = types.model("Child").props({
  flag: types.maybe(types.boolean)
});

export const StoreModel = types.model("Store").props({
  child: types.optional(ChildModel, {}) // red squiggly under `{}` with error below
});

Describe the expected behavior Before I discovered this problem when upgrading a project from much older mobx (4.2.1) & mobx-state-tree (2.0.5) versions, this worked: I could initialize MST models with naked-object snapshots and leave out maybe or optional properties if I wanted.

Describe the observed behavior If I don't specify values for all the properties when building the snapshot object to use as the default, Typescript complains:

Argument of type '{}' is not assignable to parameter of type 'OptionalDefaultValueOrFunction<IModelType<{ flag: IMaybe<ISimpleType<boolean>>; }, {}, _NotCustomized, _NotCustomized>>'.
  Type '{}' is not assignable to type '() => ExtractCSTWithSTN<IModelType<{ flag: IMaybe<ISimpleType<boolean>>; }, {}, _NotCustomized, _NotCustomized>>'.
    Type '{}' provides no match for the signature '(): ExtractCSTWithSTN<IModelType<{ flag: IMaybe<ISimpleType<boolean>>; }, {}, _NotCustomized, _NotCustomized>>'.ts(2345)

I get essentially the same error if I define ChildModel's flag as types.optional(types.boolean, false) (or just false, which is the same thing, right?). If I do child: types.optional(ChildModel, { aProp: false }) instead (with either declaration, optional or maybe), the error goes away.

[xaviergonz]

Might be related to this? https://github.com/mobxjs/mobx-state-tree/pull/1269

[kresli]

I think its related to #1307

[mweststrate]

@bryanstearns, please check your tsconfig. The compiler flags should be like:

{
    "strict": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "noImplicitAny": true,
    "noFallthroughCasesInSwitch": true,
    "noImplicitReturns": true,
    "noImplicitThis": true
}

[mweststrate]

(please note that sandbox might not pick up compiler flag changes from my experience, so best test it locally or on stackblitz)

[alex-shamshurin]

Any update on this ?