Invalid argument candidate for this type inference

[edevine]

TypeScript Version: 2.1
The infered argument candidate 'string' is not considered valid when it should be.
Code:

class MyArray<T> extends Array<T> {
    flatten<R>(this: R[][]): R[] {
        return (new Array<R>()).concat(...this);
    }
}

var a = new MyArray<string>();
a.flatten<string>(); // Desired Error

var b = new MyArray<string[]>();
b.flatten<string>(); // string[]
b.flatten(); // Inference error

Expected behavior:
b.flatten() infers the type parameter as string and returns string[]

Actual behavior:

The type argument for type parameter 'R' cannot be inferred from the usage. Consider specifying the type arguments explicitly.
Type argument candidate 'string' is not a valid type argument because it is not a supertype of candidate 'string[]'.

[mhegazy]

Seems fixed in latest.