Consider contextually typing class member functions by their base class/interface members

[kns98]

here is my code

    export enum SortDir
    {
        Asc,
        Desc,
        Flip
    }

    export interface ISortInfo
    {
        getvalue(x: QuerySummary): any; //change to number|string in TypeScript 1.4
        order: SortDir;
        ordercalc?: number;
    }

    class SortByQueryId implements ISortInfo
    {
        getvalue = x => x.QueryId;
        order = SortDir.Flip;
    }

when I type "x." I expect to see the properties of QuerySummary but I do not. VS still thinks the type is 'any'

[danquirk]

Contextual typing has only ever been applied to expressions and not statements so this has never worked. That said, it's certainly a reasonable thing to expect, at least for the function declaration style:

class SortByQueryId implements ISortInfo {
    getvalue(x) { /* x should be QuerySummary in here */ }
}

Edited the title slightly to reflect this.

[kns98]

Thanks Dan. I love this open community style development process :)

[jbrantly]

Can the scope of this be expanded to support member properties and static properties? See my comment in #1887 for an example.

[hdachev]

Adding an example of broken return value inference that results in a compiler error:

type Events = {
    [event: string]: () => void;
};

class A {
    getEvents(): Events {
        return null;
    }
}

class B extends A {
    getEvents() {
        return {
            'click': () => {

            }
        };
    }
}

// test.ts(12,7): error TS2415: Class 'B' incorrectly extends base class 'A'.
//   Types of property 'getEvents' are incompatible.
//     Type '() => { 'click': () => void; }' is not assignable to type '() => { [event: string]: () => void; }'.
//       Type '{ 'click': () => void; }' is not assignable to type '{ [event: string]: () => void; }'.
//         Index signature is missing in type '{ 'click': () => void; }'.

[omidkrad]

+1 vote for this. Related issues: #178 and #2073

I think it's not intuitive for contextual typing not to apply when overriding a class member. i.e. it should.

[j-walker23]

+1

[Coderah]

+1

[RyanCavanaugh]

Approved. When a method has multiple parent declarations (e.g. implementing multiple interfaces which each have a method named the same thing), union them to provide a contextual type.

[sandersn]

This will be fixed by #6118.

[tinganho]

Can the scope of this be expanded to support member properties and static properties? See my comment in #1887 for an example.

I'm also in support for extending it to base class members and statics.

[sandersn]

Unfortunately, we couldn't come up with a solution that was consistent and backward-compatible. See #6118 for details. I'm closing this for now.