带有使用声明链接的程序在MSVS和clang上编译，但不在GCC上编译 [英] Program with chaining of using-declarations compiles on MSVS and clang but not on GCC

问题描述

以下程序是否根据c ++标准格式正确或格式不正确？

Is the following program well-formed or ill-formed according to the c++ standard?

namespace X { int i; }

namespace Y { using X::i; }

int main() { using X::i; using Y::i; }

我用不同的编译器得到不同的结果：

I'm getting different results with different compilers:

• MSVS：编译（ http://webcompiler.cloudapp.net/ ）


• Clang：编译（ http://melpon.org/wandbox/permlink/KloDufJ5h1DalK4v ）


• GCC：编译错误（ http://melpon.org/wandbox/permlink/IKuuQGE1THofuUTr） ）

• MSVS: Compiles ( http://webcompiler.cloudapp.net/ )
• Clang: Compiles ( http://melpon.org/wandbox/permlink/KloDufJ5h1DalK4v )
• GCC: Compile error ( http://melpon.org/wandbox/permlink/IKuuQGE1THofuUTr )

我不想修复此程序，使其在GCC上编译。我只想知道c ++标准关于这一点以及为什么三个编译器的行为不同。

I don't want to fix this program to make it compile on GCC. I just want to know what the c++ standard says about this and why the three compilers behave differently. Also I want to if this is a result of a bug in any of these compilers.

推荐答案

Clang和MSVC是正确的;如果这是一个错误的结果是任何这些编译器。此代码有效。如Alf所说， [namespace.udecl]（7.3.3）/ 10 说

Clang and MSVC are correct; this code is valid. As Alf notes, [namespace.udecl] (7.3.3)/10 says

use-declaration 是一个声明 ，因此可以在允许多个声明的地方重复使用。

A using-declaration is a declaration and can therefore be used repeatedly where (and only where) multiple declarations are allowed.

但是，对块范围中的同一实体的多个声明没有限制，因此原始示例有效。不涉及 using-declaration 的相应情况是：

However, there is no restriction on multiple declarations of the same entity in block scope, so the original example is valid. A corresponding case not involving using-declarations is:

int n;
void f() {
  extern int n;
  extern int n;
}

这是有效的（并且被GCC，EDG，Clang和MSVC接受），因此（通过上面引用的规则）原始示例也是有效的。

This is valid (and is accepted by GCC, EDG, Clang, and MSVC), therefore (by the above-quoted rule) the original example is also valid.

值得注意的是， [namespace.udecl] 7.3.3）/ 10 包含错误。它说：

It is worth noting that the example in [namespace.udecl] (7.3.3)/10 contains an error. It says:

namespace A {
  int i;
}

void f() {
  using A::i;
  using A::i; // error: double declaration
}

...但注释不正确;第二个声明没有错误。请参阅核心问题36 中的讨论。我已从标准中移除示例，这样就不会混淆更多人。

... but the comment is not correct; there is no error on the second declaration. See the discussion in core issue 36. I've removed the example from the standard so that it won't confuse more people.

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