There is no ODR violation in this itself either:

  template<class T>
  class S {
  public:
    // template<std::enable_if_t<((void)sizeof(T), true), bool> = false>
    explicit S(unsigned char *) {
      static_assert(sizeof(T) > 0, "oops");
    }
  };

It was always about usage. Some constructs are easier to misuse than others.