以下代码在g++-9下会产生编译错误：

#include <ext/algorithm>
#include <algorithm>
#include <vector>

int main() {
    std::vector<unsigned int> a[3], b[3];
    copy_n(a, 3u, b);
}

错误信息：

a.cpp: In function ‘int main()’:
a.cpp:8:20: error: call of overloaded ‘copy_n(std::vector<unsigned int> [3], unsigned int, std::vector<unsigned int> [3])’ is ambiguous
    8 |     copy_n(a, 3u, b);
      |                    ^
In file included from /usr/include/c++/9/algorithm:62,
                 from /usr/include/x86_64-linux-gnu/c++/9/bits/stdc++.h:65,
                 from a.cpp:1:
/usr/include/c++/9/bits/stl_algo.h:799:5: note: candidate: ‘_OIter std::copy_n(_IIter, _Size, _OIter) [with _IIter = std::vector<unsigned int>*; _Size = unsigned int; _OIter = std::vector<unsigned int>*]’
  799 |     copy_n(_InputIterator __first, _Size __n, _OutputIterator __result)
      |     ^~~~~~
In file included from /usr/include/c++/9/ext/ropeimpl.h:47,
                 from /usr/include/c++/9/ext/rope:2975,
                 from a.cpp:2:
/usr/include/c++/9/ext/algorithm:120:5: note: candidate: ‘std::pair<_InputIterator, _ForwardIterator> __gnu_cxx::copy_n(_InputIterator, _Size, _OutputIterator) [with _InputIterator = std::vector<unsigned int>*; _Size = unsigned int; _OutputIterator = std::vector<unsigned int>*]’
  120 |     copy_n(_InputIterator __first, _Size __count, _OutputIterator __result)
      |     ^~~~~~

std::copy_n 是通过ADL查找找到的，但 __gnu_cxx::copy_n 是如何被找到的？

在使用g++-13时，代码可以编译。为什么？

解决方案

GCC11及以下版本：
std::vector<int, std::allocator<int>> -> std::allocator<int> 继承自 __gnu_cxx::new_allocator<int>（来自 <ext/new_allocator.h>），因此 __gnu_cxx 是一个关联命名空间。

GCC12：
libstdc++已修补，将 __gnu_cxx::new_allocator 改为 std::__new_allocator（在提交中 fe9571a），因此 __gnu_cxx 不再是任何具有 std::allocator 的对象的关联命名空间。