flowtype：$ Subtype似乎已经破坏了类

Question

我认为以下情况应该失败：

class A { a: boolean; }
class B extends A { b: boolean; }

// this works, thus "proving" B is a subtype of A
const a: A = new B();

// however, this also works! It should fail.
class C { c: boolean; } // a C cannot be assigned to A, it's not a subtype
type SubtypeAcceptor<T> = ($Subtype<T>) => boolean;
const a2: SubtypeAcceptor<A> = (c: C) => (typeof c === 'object');

是什么给出的？上述的非通用版本也会失败：

type SubtypeAcceptorOfA = ($Subtype<A>) => boolean;
const a3: SubtypeAcceptorOfA = (c: C) => (typeof c === 'object');

这是因为理论上可以C & $Subtype<A>吗？上面的确切对象类型也失败如下：

const a4: SubtypeAcceptor<{| a: boolean |}> = (c: {| b: boolean |}) => c.b;

我认为{| b: boolean |}显然不能是{| a: boolean |}的子类型。