Order of method overloading declaration matters when one parameter type extends the other

[hraban]

Playing around with the type definitions for React 0.13 I ran into this problem:

declare module Test {
    interface Parent {
        y: number;
    }
    interface Child extends Parent {
        z: string;
    }

    interface ParentDeriv {
        yderiv: number;
    }
    interface ChildDeriv extends ParentDeriv {
        zderiv: string;
    }

    interface Api {
        makeParent(): Parent;
        makeChild(): Child;
        meth(element: Parent): ParentDeriv;
        meth(element: Child): ChildDeriv;
    }

    var api: Api;
}

var x = Test.api.meth(Test.api.makeParent());
x.zderiv; // Should be: error. Is: error.
var y = Test.api.meth(Test.api.makeChild()); // takes wrong method.
y.zderiv; // Should be: ok. Is: error. y is inferred ParentDeriv.

But if you flip the method declarations around, it works fine:

declare module Test {
    interface Parent {
        y: number;
    }
    interface Child extends Parent {
        z: string;
    }

    interface ParentDeriv {
        yderiv: number;
    }
    interface ChildDeriv extends ParentDeriv {
        zderiv: string;
    }

    interface Api {
        makeParent(): Parent;
        makeChild(): Child;
        // Note order: child class first
        meth(element: Child): ChildDeriv;
        meth(element: Parent): ParentDeriv;
    }

    var api: Api;
}

var x = Test.api.meth(Test.api.makeParent());
x.zderiv; // Should be: error. Is: error.
var y = Test.api.meth(Test.api.makeChild());
y.zderiv; // Should be: ok. Is: ok.

This is a bug, right? With TS I'm never sure..

Tested with 1.4 (Playground and local)

[RyanCavanaugh]

Overload ordering absolutely matters. The rule is that the first applicable overload is chosen. We tried a bunch of other rules and this was the only one anyone could ever make sense of in practice.

This does mean that it's possible to write overloads which are "unreachable" by putting the general signature before the more specific overload; care needs to be taken to put the more-specific overloads first.

[hraban]

Alright thanks for getting back so quickly, sorry for the false alarm.