Generic type check seems cannot infer type

[Delagen]

TypeScript Version: 2.7.0-dev.201xxxxx

Search Terms:

Code

const property: keyof IArrayItem = "test";

interface IArrayItem {
    test: string;
}

function A<T extends IArrayItem = IArrayItem>(array: T[]) {
    const t: Partial<T> = { test: "aaa" }; // Error: Type '{ test: "aaa"; }' is not assignable to type 'Partial<T>'.
    const t2: Partial<IArrayItem> = { test: "aaa" }; // Works
}

const array: IArrayItem[] = [];
A(array);

Expected behavior:
No type check errors

Actual behavior:
Error: Type '{ test: "aaa"; }' is not assignable to type 'Partial'

Playground Link: playground

Related Issues:

[Delagen]

See also this discussion DefinitelyTyped/DefinitelyTyped@64104e9

[jack-williams]

I think this should be an error. There is no guarantee that the object literal is assignable for all possible Partial<T>. A caller is free to instantiate the generic in A to be the following type T.

type T = { test: "foo" };
type Extends<T extends IArrayItem> = T // Prove T extends IArrayItem
const example: Partial<Extends<T>> = { test: "aaa" }; // Error

[ahejlsberg]

This is working as intended and is a duplicate of many existing issues, e.g. #19388, #17071, #13442.