c++ - 继承默认构造函数在 gcc 中失败，但在 clang 中有效，哪个有 bug？

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;

据我通过阅读 cppreference 了解到的, Derived应该继承默认的Base::Base()构造函数和上面的代码应该可以顺利编译。

编辑:我的错，我链接到的页面讲述了完全相反的故事。所以，clang 似乎出现了回归。

但是，我尝试过的所有 gcc 版本都失败了，提示 Derived没有默认构造函数，而 clang 则可以，但仅从版本 3.9.0 开始； g++-7 段错误，甚至 ¹。

你可以在godbolt上自行查看.

那么，这是谁的错呢？ Clang 允许它，还是 gcc(排除段错误)不允许它？

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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。您应该报告这一点。

其次，默认构造函数永远不会被继承。来自N3242(N3797中的措辞类似)，[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 的默认构造函数不会继承到 Derived 中，因此 Derived t 应该是格式错误的，因为没有有效的构造函数采用零参数。

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在 C++17 中，尽管措辞不同，但这仍然是错误的。还是 [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。但没有 Derived::Derived()。