返回类型为'x = unit的方法不能覆盖泛型抽象方法

Question

在下面的代码中,UnitClass1.F似乎覆盖BaseClass1<unit>.F,但实际上没有.有人能说出原因吗？

// Method case.
type [<AbstractClass>] BaseClass1<'x>() =
  abstract member F: unit -> 'x

type UnitClass1() =
  inherit BaseClass1<unit>()

  // ERROR: F: unit -> unit doesn't have the correct type to override the corresponding abstract method.
  override this.F() = ()

  // ERROR: Wrong number of parameters.
  //override this.F(()) = ()

  // ERROR: UnitClass1 doesn't have an implementation for abstract member F: unit -> 'x
  //member this.F() = ()

属性上也会出现此问题.

// Property case.
type [<AbstractClass>] BaseClass2<'x>() =
  abstract member P: 'x

type UnitClass2() =
  inherit BaseClass2<unit>()

  // The same error as the above.
  member this.P = ()

但是,如果我将泛型参数移动到参数侧,那么该代码将编译.

type [<AbstractClass>] BaseClass3<'x>() =
  abstract member F: 'x -> unit 

type UnitClass3() =
  inherit BaseClass3<unit>()

  // OK
  override this.F(()) = ()

当然,如果'x不是,他们会编译unit.

type [<AbstractClass>] BaseClass4<'x>() =
  abstract member P: 'x

type UnitClass4() =
  inherit BaseClass4<int>()

  // OK
  override this.P = 0

环境:Windows 10,F#4.0,Visual Studio 2015社区