继承默认构造函数在 gcc 中失败并在 clang 中工作，哪个有错误？

Question

举这个简单的例子。

struct Base {
    // Base::Base() defined by the compiler
};

struct Derived: Base { 
    using Base::Base; // Should inherit Base::Base()

    Derived(int value):
        m_value(value)
    {}

private:
    int m_value; // If Base::Base() is invoked, it's default constructed
};

Derived t;

~~据我阅读 cppreference 的理解，Derived 应该继承默认的 Base::Base() 构造函数，上面的代码应该可以愉快地编译。~~

编辑：糟糕，我链接到的页面讲述了完全相反的故事。所以看起来 clang 有一个回归。

然而，我尝试过的所有 gcc 版本都失败了，抱怨 Derived 没有默认构造函数，而 clang 做得很好，但 仅从 3.9.0 版开始; g++-7 segfaults, even¹.

大家可以在godbolt上自行查看。

那么，这是谁的错？允许它的 Clang，或者不允许它的 gcc（禁止段错误）？

¹ 虽然似乎只在 godbolt 上这样做，但我无法在本地重现段错误。

Answer 1

首先，编译器段错误始终是编译器错误。你应该报告那个。

其次，默认构造函数永远不会被继承。来自N3242（N3797中的写法类似），[class.inhctor]:

For each non-template constructor in the candidate set of inherited constructors other than a constructor having no parameters or a copy/move constructor having a single parameter, a constructor is implicitly declared with the same constructor characteristics unless there is a user-declared constructor with the same signature in the class where the using-declaration appears.

Base 的默认构造函数未继承到 Derived，因此 Derived t 应该是 ill-formed，因为没有采用零参数的有效构造函数。

在 C++17 中，这仍然是 ill-formed，尽管措辞不同。仍然 [class.inhctor]，来自 N4618：

When a constructor for type B is invoked to initialize an object of a different type D (that is, when the constructor was inherited (7.3.3)), initialization proceeds as if a defaulted default constructor were used to initialize the D object and each base class subobject from which the constructor was inherited, except that the B subobject is initialized by the invocation of the inherited constructor.

要调用 Base::Base()，我们必须从 Derived::Derived() 开始。但是没有Derived::Derived()。

继承默认构造函数在 gcc 中失败并在 clang 中工作，哪个有错误？

Inheriting default constructor fails in gcc and works in clang, which one's got the bug?

c++

inheritance

g++

c++11

clang++