==== tests/cases/compiler/generics4.ts (1 errors) ====
    class C<T> { private x: T; }
    interface X { f(): string; }
    interface Y { f(): boolean; }
    var a: C<X>;
    var b: C<Y>;
    
    a = b; // Not ok - return types of "f" are different
    ~
!!! Type 'C<Y>' is not assignable to type 'C<X>':
!!!   Type 'Y' is not assignable to type 'X':
!!!     Types of property 'f' are incompatible:
!!!       Type '() => boolean' is not assignable to type '() => string':
!!!         Type 'boolean' is not assignable to type 'string'.