==== tests/cases/compiler/genericArrayExtenstions.ts (1 errors) ====
    export declare class ObservableArray<T> implements Array<T> { // MS.Entertainment.ObservableArray
                         ~~~~~~~~~~~~~~~
!!! genericArrayExtenstions.ts(1,22): error TS2137: Class ObservableArray<T> declares interface T[] but does not implement it:
!!! 	Type 'ObservableArray<T>' is missing property 'join' from type 'T[]'.
    concat<U extends T[]>(...items: U[]): T[];
    concat(...items: T[]): T[];
    }
    