==== tests/cases/compiler/covariance1.ts (2 errors) ====
    module M {
    
        interface X { m1:number; }
        export class XX implements X { constructor(public m1:number) { } }
                                   ~
!!! covariance1.ts(4,32): error TS2019: Exported class 'XX' implements private interface 'X'.
    
        interface Y { x:X; }
    
        export function f(y:Y) { }
                          ~~~
!!! covariance1.ts(8,23): error TS2040: Parameter 'y' of exported function has or is using private type 'Y'.
    
        var a:X;
        f({x:a}); // ok
    
        var b:XX;
        f({x:b}); // ok covariant subtype
    }
    
    