==== tests/cases/compiler/classImplementsClass4.ts (2 errors) ====
    class A {
        private x = 1;
        foo(): number { return 1; }
    }
    class C implements A {
          ~
!!! classImplementsClass4.ts(5,7): error TS2137: Class C declares interface A but does not implement it:
!!! 	Type 'C' is missing property 'x' from type 'A'.
        foo() {
            return 1;
        }
    }
    
    class C2 extends A {}
    
    var c: C;
    var c2: C2;
    c = c2;
    c2 = c;
    ~~
!!! classImplementsClass4.ts(16,1): error TS2012: Cannot convert 'C' to 'C2':
!!! 	Type 'C' is missing property 'x' from type 'C2'.