=== tests/cases/compiler/functionOverloads23.ts ===
function foo(bar:(b:string)=>void);
>foo : { (bar: (b: string) => void): any; (bar: (a: number) => void): any; }, Symbol(foo, Decl(functionOverloads23.ts, 0, 0), Decl(functionOverloads23.ts, 0, 35), Decl(functionOverloads23.ts, 1, 35))
>bar : (b: string) => void, Symbol(bar, Decl(functionOverloads23.ts, 0, 13))
>b : string, Symbol(b, Decl(functionOverloads23.ts, 0, 18))

function foo(bar:(a:number)=>void);
>foo : { (bar: (b: string) => void): any; (bar: (a: number) => void): any; }, Symbol(foo, Decl(functionOverloads23.ts, 0, 0), Decl(functionOverloads23.ts, 0, 35), Decl(functionOverloads23.ts, 1, 35))
>bar : (a: number) => void, Symbol(bar, Decl(functionOverloads23.ts, 1, 13))
>a : number, Symbol(a, Decl(functionOverloads23.ts, 1, 18))

function foo(bar:(a?)=>void) { return 0 }
>foo : { (bar: (b: string) => void): any; (bar: (a: number) => void): any; }, Symbol(foo, Decl(functionOverloads23.ts, 0, 0), Decl(functionOverloads23.ts, 0, 35), Decl(functionOverloads23.ts, 1, 35))
>bar : (a?: any) => void, Symbol(bar, Decl(functionOverloads23.ts, 2, 13))
>a : any, Symbol(a, Decl(functionOverloads23.ts, 2, 18))
>0 : number