interface Gen<T> {
   method1 (): void
}
// not use '='? it is a break change
interface Gen<T = number> {
   method2 (): void
}

let v1 = {} as Gen<string>
v1.method1() // correct ,  Gen<string> has method1
v1.method2() // error,  Gen<string> does not has method2

let v2 = {} as Gen<number>
v1.method2() // correct ,  Gen<string> has method2