i3.ts(6,1): error TS2322: Type 'I3' is not assignable to type '{ one: number; }'.
  Types of property 'one' are incompatible.
    Type 'number | undefined' is not assignable to type 'number'.
      Type 'undefined' is not assignable to type 'number'.


==== i3.ts (1 errors) ====
    interface I3 { one?: number; }; 
    declare var x: {one: number};
    declare var i: I3;
    
    i = x;
    x = i;
    ~
!!! error TS2322: Type 'I3' is not assignable to type '{ one: number; }'.
!!! error TS2322:   Types of property 'one' are incompatible.
!!! error TS2322:     Type 'number | undefined' is not assignable to type 'number'.
!!! error TS2322:       Type 'undefined' is not assignable to type 'number'.