引用规则折叠会导致这种行为吗？

Is it a rule of reference collapsing that causes this behaviour?

是。您有：

T = int&
const T& = const (int&) &

引用不能为 const （您仍然无法重新绑定它们，因此将其忽略），对引用的引用只是引用。

References can't be const (you can't rebind them anyways, so it's ignored) and a reference to a reference is just a reference.

所以您拥有

const T& = int&

要解决此问题，您需要应用 const 到基础类型，您可以通过删除引用来实现：

To fix this, you need to apply const to the underlying type, which you can do like this by removing the reference:

const std::remove_reference_t<T>& get() { return x_; }
//    ^^^^^^^^^^^^^^^^^^^^^^^