tests/cases/compiler/extendGenericArray2.ts(8,5): error TS2322: Type 'string' is not assignable to type 'number'. ==== tests/cases/compiler/extendGenericArray2.ts (1 errors) ==== interface IFoo { x: T; } interface Array extends IFoo { } var arr: string[] = []; var y: number = arr.x; ~ !!! error TS2322: Type 'string' is not assignable to type 'number'.