tests/cases/compiler/functionOverloads.ts(4,13): error TS2345: Argument of type 'number' is not assignable to parameter of type 'string'. ==== tests/cases/compiler/functionOverloads.ts (1 errors) ==== function foo(): string; function foo(bar: string): number; function foo(bar?: string): any { return "" }; var x = foo(5); ~ !!! error TS2345: Argument of type 'number' is not assignable to parameter of type 'string'.