仅在GCC和Clang中进行了测试，基类中存在传递值复制赋值运算符（在实现复制和交换（或复制和- move）习惯用法）导致隐式删除派生类中的副本分配运算符。

Clang和GCC对此表示同意； 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;
}