Problem:
trait UpperBound[O]
trait High[F[O] <: UpperBound[O]]
def canEqual(that :Any) = that.isInstanceOf[High[_]]
def high(h :High[_]) = ???
无法编译，因为 scalac 看到_类型而不是它期望的类型构造函数。如何解决这个问题，最好不写小说？
原始问题（在编辑回复 Dmytro 的答案之前）有：
def canEqual(that :Any) = that.isInstanceOf[High[m forSome { type m[O] <: UpperBound[O] }]]
def high(h :High[m forSome { type m[O] <: UpperBound[O] }] = ???
是否有使用通配符表达式来编写上述两种方法的更短方法？
只需使用_ in High的类型参数位置不起作用，因为类型不匹配，并且_[_]甚至不是有效的类型表达式。
-
如果你在外面进行存在量化High那么就只是
type T = High[F] forSome { type F[O] <: UpperBound[O] }
def canEqual(that: Any) = that.isInstanceOf[T]
def high(h: T) = ???
-
如果你在里面进行存在量化High然后自从
implicitly[(n forSome { type n <: Upper}) =:= Upper]
implicitly[(m[O1] forSome { type m[O] <: UpperBound[O]}) =:= UpperBound[O1]]
（反之亦然）这只是High[UpperBound]
implicitly[High[m forSome { type m[O] <: UpperBound[O] }] =:= High[UpperBound]]
def canEqual(that: Any) = that.isInstanceOf[High[UpperBound]]
def high(h: High[UpperBound]) = ???
存在主义类型???? forSome { ???? } where ????包含子句类型????[tps]>:????<:????相当于类型????′ forSome { ???? } where ????′由于。。。导致的结果????通过替换每个协变出现???? in ???? by ????并通过替换每个逆变出现???? in ???? by ????.
https://scala-lang.org/files/archive/spec/2.13/03-types.html#simplification-rules
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