编译器差异：别名解析和名称查找之间的交互

Question

考虑这段代码：

using type = long;

namespace n {
  using type = long;
}

using namespace n;
int main() {
  type t;
}

这在 Clang 3.7 and GCC 5.3, but MSVC 19* 上编译得很干净，给出了以下错误消息：

main.cpp(9): error C2872: 'type': ambiguous symbol
main.cpp(1): note: could be 'long type'
main.cpp(4): note: or       'n::type'

这段代码格式正确吗？标准的哪一部分说明在歧义检查之前是否解析了别名？

请注意，如果您更改其中一个别名，Clang 和 GCC 都会给出与 MSVC 类似的错误。

我完全知道限定名称将如何解决歧义，我只是对标准对此有何评论感兴趣。

*- 粘贴代码然后运行到link，我不知道是否有带有permalinks

的在线MSVC编译器

Answer 1

[namespace.udir]/6:

If name lookup finds a declaration for a name in two different namespaces, and the declarations do not declare the same entity and do not declare functions, the use of the name is ill-formed.

但是，它们确实声明了引用相同类型的名称，因此程序应该是合式的。这种解释是例如由 core issue 1894 中的评论确认：

  //[..]

  namespace C {
    // The typedef does not redefine the name S in this
    // scope, so issue 407's resolution does not apply.
    typedef A::S S;
    using A::S;
    // **The name lookup here isn't ambiguous, because it only finds one
    // entity**, but it finds both a typedef-name and a non-typedef-name referring
    // to that entity, so the standard doesn't appear to say whether this is valid.
    struct S s;
  }

Answer 2

7.3.4 / 6:

If name lookup finds a declaration for a name in two different namespaces, and the declarations do not declare the same entity and do not declare functions, the use of the name is ill-formed

编译器差异：别名解析和名称查找之间的交互

Compiler discrepancy: Interaction between alias resolution and name lookup

c++

typedef

language-lawyer

name-lookup

c++11