=== tests/cases/compiler/functionOverloads9.ts ===
function foo(foo:string);
>foo : Symbol(foo, Decl(functionOverloads9.ts, 0, 0), Decl(functionOverloads9.ts, 0, 25))
>foo : Symbol(foo, Decl(functionOverloads9.ts, 0, 13))

function foo(foo?:string){ return '' };
>foo : Symbol(foo, Decl(functionOverloads9.ts, 0, 0), Decl(functionOverloads9.ts, 0, 25))
>foo : Symbol(foo, Decl(functionOverloads9.ts, 1, 13))

var x = foo('foo');
>x : Symbol(x, Decl(functionOverloads9.ts, 2, 3))
>foo : Symbol(foo, Decl(functionOverloads9.ts, 0, 0), Decl(functionOverloads9.ts, 0, 25))