使用 vs typedef pointer-to-noexcept-function 不一致

Question

我意识到我可以通过 using 为指向 noexcept 函数的指针声明一个类型，但是如果我使用 typedef，我将被禁止这样的声明。考虑以下代码：

#include <iostream>

using fptr = void(*)() noexcept;
// typedef void(*FPTR)() noexcept; // fails to compile

void f() noexcept
{
    std::cout << "void f() noexcept" << std::endl;
}

void g()
{
    std::cout << "void g()" << std::endl;
    throw 10;
}

int main()
{
    fptr f1 = f;
    fptr f2 = g; // why can we do this?

    try {
        f1();
        f2();
    }
    catch (...)
    {
        std::cout << "Exception caught" << std::endl;
    }
}

如果我取消注释 FPTR 声明，我会得到

error: 'FPTR' declared with an exception specification

然而，using 工作正常。使用 gcc4.9 和 gcc5 编译。

我的问题是：

1. 为什么会出现这种不一致？
2. 为什么我们甚至可以将 using 与 noexcept 一起使用，因为我们可以将指针绑定到未声明 [​​=11=] 的函数，如行 fptr f2 = g;

---

好像是gcc相关的bug，连gcc5都抓不到。填写错误报告

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65382

Answer 1

这似乎是 GCC 中的一个错误； Clang 拒绝两者，因为类型别名或 typedefs 中不允许异常规范。这可以在以下位置找到：

15.4 Exception specifications [except.spec]

² An exception-specification shall appear only on a function declarator for a function type, pointer to function type, reference to function type, or pointer to member function type that is the top-level type of a declaration or definition, or on such a type appearing as a parameter or return type in a function declarator. An exception-specification shall not appear in a typedef declaration or alias-declaration.

[...]

标准还明确表示两者应该一致：

7.1.3 The typedef specifier [dcl.typedef]

² A typedef-name can also be introduced by an alias-declaration. The identifier following the using keyword becomes a typedef-name and the optional attribute-specifier-seq following the identifier appertains to that typedef-name. It has the same semantics as if it were introduced by the typedef specifier. In particular, it does not define a new type and it shall not appear in the type-id.

（强调我的）

回答你的第二个问题：GCC 可能只是忽略了异常规范，因为不能仅在异常规范上重载函数——它不是函数签名的一部分。可以在这里找到：

8.3.5 Functions [dcl.fct]

⁶ [...] The return type, the parameter-type-list, the ref-qualifier, and the cv-qualifier-seq, but not the default arguments (8.3.6) or the exception specification (15.4), are part of the function type. [ Note: Function types are checked during the assignments and initializations of pointers to functions, references to functions, and pointers to member functions. — end note ]