noexcept，继承构造函数和对实际上完整的不完整类型的无效使用

Question

我不确定这是 GCC 编译器的错误还是 noexcept.
的预期行为考虑以下示例：

struct B {
    B(int) noexcept { }
    virtual void f() = 0;
};

struct D: public B {
    using B::B;
    D() noexcept(noexcept(D{42})): B{42} { }
    void f() override { }
};

int main() {
    B *b = new D{};
}

如果删除 noexcept，它会编译。
无论如何，就像在示例中一样，我从 GCC v5.3.1 得到了这个错误：

test.cpp:8:31: error: invalid use of incomplete type ‘struct D’
     D() noexcept(noexcept(D{42})): B{42} { }
                               ^

据我所知，struct D 不是一个不完整的类型，但是声明中涉及继承构造函数，看起来编译器实际上正在考虑基本结构的完整性 B超过 D.

这是预期行为还是合法代码？

为了清楚起见：

here 使用 clang 3.7.1
here 使用 GCC 5.3.0

有关 GCC 编译器的 bugzilla，请参阅 this link 了解更多详细信息。
目前，该错误仍未得到证实。我会尽快更新问题。

Answer 1

您的代码是合法的，即使 GCC 另有声明。它冒犯了这个看起来很滑稽的声明：

D() noexcept(noexcept(D{42}));

最外层的 noexcept 是一个 noexcept specifier, stating that D::D() is noexcept if and only if its constant-expression argument evaluates to true. The inner noexcept is a noexcept operator，它在编译时检查其参数表达式（实际上未计算）是否不抛出异常。因为D::D(int)是noexcept（继承自B），所以应该是这样。

cppreference.com 明确指出允许在说明符内使用运算符（强调已添加）：

The noexcept operator performs a compile-time check that returns true if an expression is declared to not throw any exceptions.

It can be used within a function template's noexcept specifier to declare that the function will throw exceptions for some types but not others.

现在，由于标准的 §9.2.2（添加了粗体强调），在 noexcept 说明符中 class 应被视为 complete：

A class is considered a completely-defined object type (3.9) (or complete type) at the closing } of the class-specifier. Within the class member-specification, the class is regarded as complete within function bodies, default arguments, using-declarations introducing inheriting constructors (12.9), exception-specifications, and brace-or-equal-initializers for non-static data members (including such things in nested classes). Otherwise it is regarded as incomplete within its own class member-specification.

§15.4.1 将 异常规范 定义为以下语法：

exception-specification:

dynamic-exception-specification

noexcept-specification

所以 GCC 不应该拒绝你的代码。

noexcept，继承构造函数和对实际上完整的不完整类型的无效使用

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