=== tests/cases/compiler/functionOverloads8.ts ===
function foo();
>foo : { (): any; (foo: string): any; }

function foo(foo:string);
>foo : { (): any; (foo: string): any; }
>foo : string

function foo(foo?:any){ return '' }
>foo : { (): any; (foo: string): any; }
>foo : any
>'' : string