Empty object is not assignable to type argument with constrained index signature

[tinganho]

interface B {
    [index: string]: string;
}
class A<T extends B> {
    a: T = {}; // Error
    b = {} as T; // No error
}

let b: B = {}; // No error

[DanielRosenwasser]

You can't assign a {} to a T because you don't know whether a consumer of A will instantiate T with some more derived type. So for instance, let's add to your example:

interface B {
    [index: string]: string;
}
class A<T extends B> {
    a: T = {}; // assume this will be okay
}

interface M extends B {
    abc: string;
    def: number;
}

// 'a' should have type 'M',
// but your class set its value to '{}'!
let a = new A<M>().a;

So it's not appropriate to allow this.

[tinganho]

Ok thanks.