Generic parameters inference in interface

[MasGaNo]

TypeScript Version: 3.4.0-dev.20190226

Search Terms:
Generic parameters inference

Code

interface IFoo {
    id: string;
}

interface ITest<T extends IFoo | IFoo[]> {
    data: T;
};

interface ITest2<T extends IFoo | IFoo[]= IFoo> {
    data: T;
}

const test1: ITest = { // Generic type 'ITest<T>' requires 1 type argument(s). ts(2314)
    data: [{
        id: ''
    }]
}

const test2: ITest2 = {
    data: [{ // Property 'id' is missing in type '{ id: string; }[]' but required in type 'IFoo'. ts(2741)
        id: 'fds'
    }]
}

Expected behavior:
I expect the interface ITest to understand that data is typeof Array<IFoo> like for generic method inference.
If I precise a default value for generic parameter (ITest2), I can understand that in my test2 implementation, it will take IFoo as a default implementation, even if I found the error explanation a little bit weird to understand.

Actual behavior:
As describe in the code section, I get some errors because I must provide the expected type.
The thing is, sometime, in some third party library definition, they don't export some type, so I cannot easily pick the expected variant of the generic type I want to have.
A way to fix it, is to recreate a type by mimicking the structure of the original type, but I increase the number of source of truth, so as soon as the third party update the definition, it breaks my duplicated type.

Playground Link:
Playground link

Related Issues:
Well... I found lot of issues but related with generic method (that works like a charm today), not really just for interface...

[RyanCavanaugh]

Working on this area at #26242

[MasGaNo]

Oh! true, I miss this one!
Great news! I'm looking forward this issue.

[ianstormtaylor]

Still would really love to see this for defining plain objects that map to a generic type! Would be really helpful for creating inputs that a framework expects without having to use extra factory functions everywhere.