tcpoly_variance.scala

来自「JAVA 语言的函数式编程扩展」· SCALA 代码 · 共 7 行

class A[m[x]] {
 def str: m[Object] = error("foo")
}

class B[m[x]] extends A[m] {
 override def str: m[String]  = error("foo") // since x in m[x] is invariant, ! m[String] <: m[Object]
}