Overloads returning type predicate types are always incompatible with implementation

[DanielRosenwasser]

Version 1.6.0-dev.20150805

function f(x: string): x is string;
function f(x: number): x is number;
function f(x: any): boolean {
    return true;
}

Currently yields

blah.ts(1,10): error TS2394: Overload signature is not compatible with function implementa
tion.

I think this should be fixed in time for 1.6; being able to do this would enable some useful patterns.

[DanielRosenwasser]

Specific use case found with @roganov here.

namespace ActionType {
    export type Foo = string & { _fooTag: any };
    export const Foo = <Foo>"Foo";

    export type Bar = string & { _barTag: any };
    export const Bar = <Bar>"Bar";
}

type ActionType = ActionType.Foo |
                  ActionType.Bar;


interface BaseAction {
    type: ActionType;
}

interface FooAction extends BaseAction {
    type: ActionType.Foo;
    foo: number;
}

interface BarAction extends BaseAction {
    type: ActionType.Bar;
    bar: string;
}

function isActionType(action: BaseAction, type: ActionType.Foo): action is FooAction;
function isActionType(action: BaseAction, type: ActionType.Bar): action is BarAction;
function isActionType(action: BaseAction, type: ActionType): boolean {
    return action.type === type;
}

let handleAction = (action: BaseAction) => {
    if (isActionType(action, ActionType.Foo)) {
        let foo = action.foo; // OK
        let bar = action.bar; // type error
    }
    else if (isActionType(action, ActionType.Bar)) {
        let bar = action.bar; // OK
    }
}

[tinganho]

See #3262 (comment).

[DanielRosenwasser]

Gotcha - but there was no formal issue filed for that, correct?

[tinganho]

Correct, at least non that I knew of.

[DanielRosenwasser]

@JsonFreeman in the comment that @tinganho linked to, you mentioned that this is a problem that "we've known about". What problem were you referring to specifically?

[JsonFreeman]

I was referring to #943.

[DanielRosenwasser]

Right, I was sure we had an issue referring to that. Glad I'm not going nuts 😄. @mhegazy.