=== tests/cases/compiler/functionOverloads23.ts ===
function foo(bar:(b:string)=>void);
>foo : { (bar: (b: string) => void): any; (bar: (a: number) => void): any; }
>bar : (b: string) => void
>b : string

function foo(bar:(a:number)=>void);
>foo : { (bar: (b: string) => void): any; (bar: (a: number) => void): any; }
>bar : (a: number) => void
>a : number

function foo(bar:(a?)=>void) { return 0 }
>foo : { (bar: (b: string) => void): any; (bar: (a: number) => void): any; }
>bar : (a?: any) => void
>a : any