#include <cassert>
#include <type_traits>

class Rule {
protected:
    Rule() = default;
    Rule(const Rule&) = default;
    Rule& operator=(const Rule&) = default;
public:
    virtual ~Rule() = default;
    virtual int amount() const noexcept = 0;
};

class FixedRule final : public Rule {
    int amount_;
public:
    explicit FixedRule(int amount) : amount_(amount) {}
    int amount() const noexcept override { return amount_; }
};

static_assert(!std::is_assignable<Rule&, const Rule&>::value,
              "The interface does not support assignment");
static_assert(std::is_copy_assignable<FixedRule>::value,
              "Concrete values remain assignable");

int main() {
    FixedRule first(7);
    const FixedRule second(19);
    first = second;
    const Rule& view = first;
    assert(view.amount() == 19);
    assert(second.amount() == 19);
}