=== tests/cases/compiler/functionOverloads9.ts ===
function foo(foo:string);
>foo : (foo: string) => any
>foo : string

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

var x = foo('foo');
>x : any
>foo('foo') : any
>foo : (foo: string) => any
>'foo' : string