Cannot infer right type from overload functions

[k8w]

TypeScript Version: 2.4.0 / nightly (2.5.0-dev.201xxxxx)
2.4.0

Code

interface Test{
    show(): string;
    show(arg: (a:string)=>void): void;
    show(arg: (a:string, b:number)=>void): void;
}

declare let t:Test;

t.show();
t.show(a=>{});
t.show((a,b)=>{});

Expected behavior:
Compile success.
Both a=>{} and (a,b)=>{} can infer right type to a and b instead of implicitly has an 'any' type

Actual behavior:

index.ts(11,9): error TS7006: Parameter 'a' implicitly has an 'any' type.
index.ts(11,11): error TS7006: Parameter 'b' implicitly has an 'any' type.

[kitsonk]

Don't over complicated things:

interface Test{
    show(arg?: (a:string, b?:number)=>void): void;
}

declare let t:Test;

t.show();
t.show(a=>{});
t.show((a, b) => { });

[RyanCavanaugh]

Duplicate #11936

Note: Don't mark callback parameters as optional

interface Test{
    show(arg?: (a:string, b:number)=>void): void;
}

declare let t:Test;

t.show();
t.show(a=>{});
t.show((a, b) => { });

[kitsonk]

Note: Don't mark callback parameters as optional

Ooops, sorry, yes I forgot.