为什么基类中的复制和交换会导致在派生类中隐式删除复制赋值运算符？

问：

仅在 GCC 和 Clang 中测试，基类中存在按值传递的复制赋值运算符（在实现复制和交换（或复制和移动）惯用语时很有用）会导致派生类中的复制赋值运算符被隐式删除。

Clang 和 GCC 对此达成一致;为什么会这样？

示例代码：

#include <string>
#include <iostream>

struct base {
    base() {
        std::cout << "no-arg constructor\n";
    }
    base(const base& other) :
        str{other.str} {
        std::cout << "copy constructor\n";
    }
    base(base&& other) :
        str{std::move(other.str)} {
        std::cout << "move constructor\n";
    }
    base& operator=(base other) {
        std::cout << "copy assigment\n";
        str = std::move(other.str);
        return *this;
    }
    base& operator=(base&& other) {
        std::cout << "move assigment\n";
        str = std::move(other.str);
        return *this;
    }

    std::string str;
};

struct derived : base {
    derived() = default;
    derived(derived&&) = default;
    derived(const derived&) = default;
    derived& operator=(derived&&) = default;
    derived& operator=(const derived&) = default;
};

derived foo() {
    derived ret;
    ret.str = "Hello, world!";
    return ret;
}

int main(int argc, const char* const* argv) {

    derived a;
    a.str = "Wat";
    a = foo(); // foo() returns a temporary - should call move constructor
    return 0;
}