Fields of class generic type constraint are not assignable anything but null or undefined without assertion

[seangwright]

TypeScript Version: 2.4.1

Code
Typescript Playground

interface State {
    isReady: false;
}

// Does not work
class Base1<T extends State> {
    myState: Readonly<T> = { isReady: false }; // Nope
    myOtherState: T = { isReady: false }; // Nope
    mySecondState: T = null; // Sure!
    theThirdEstate: T = undefined // Ok!

    setState(state: Partial<T>) {
        this.myState = Object.assign(this.myState, state);
    }

    update() {
        this.setState({ isReady: false }); // Nope
    }
}

// Works with 'as T'
class Base2<T extends State> {
    myState: Readonly<T> = { isReady: false } as T;
    myOtherState: T = { isReady: false } as T;

    setState(state: Partial<T>) {
        this.myState = Object.assign(this.myState, state);
    }

    update() {
        this.setState({ isReady: false } as T);
    }
}

Expected behavior:
The constraint of T as having a property isReady is flowed through and { isReady: false } is considered to have the same shape as T and is valid for T, Partial<T> or Readonly<T> without having use the explicit type assertion.

Actual behavior:
Manual type assertion is required but then compiles. Null and undefined are the only valid values to assign these class fields.

I suppose I could also replace all references to T outside of the generic constraint with State but that's no fun.

[ikatyang]

See #16985 (comment)

And if you enable the --strictNullChecks option, you'll see errors on null and undefined.

[seangwright]

Thanks for the heads up - wouldn't have found that issue myself. I'll follow along there - this issue can be closed if needed.

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.