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.

• I've checked documentation and searched for existing issues
• I've made sure my project is based on the latest MST version
• 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?
#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 ?

[thegedge]

I'm not seeing this locally

A lot has changed in the TS definitions for MST lately, along with TS upgrades. Let me know if anyone is still seeing this and, if so, the full set of relevant configs.

[coolsoftwaretyler]

Thanks for the follow up @thegedge! I do not see this error on v6 either, so I'm going to close it out.

For everyone else in the thread, sorry this has stayed open for so long, although we have definitely had a fix out for a little while.

If y'all need anything else, we can always reopen. Have a great day!