我写了以下简单的C++程序。
#include <iostream>
using namespace std;
int main() {
cout << "Hello, World" << endl;
return 0;
当我用g++编译时，它工作得很好。当我试图用Clang++编译时，我得到以下错误。
main.cpp:1:10: fatal error: 'iostream' file not found
#include <iostream>
^~~~~~~~~~
1 error generated.
用-v参数运行，我看到了以下情况。
ignoring nonexistent directory "/usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/backward"
ignoring nonexistent directory "/include"
ignoring duplicate directory "/usr/include/clang/6.0.0/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++
/usr/include/clang/6.0.0/include
/usr/local/include
/usr/include/x86_64-linux-gnu
/usr/include
End of search list.
逐一查看这些文件夹，我发现在/usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++中（或者更简洁地说，在/usr/include/c++中）我有以下目录。
drwxr-xr-x 5 root root 4.0K Feb 4 09:38 .
drwxr-xr-x 101 root root 20K Feb 4 12:22 ..
drwxr-xr-x 12 root root 12K May 24 2018 5
drwxr-xr-x 12 root root 12K Oct 9 14:53 7
drwxr-xr-x 5 root root 4.0K Feb 4 09:38 v1
lrwxrwxrwx 1 root root 1 Apr 11 2018 5.5.0 -> 5
lrwxrwxrwx 1 root root 1 Apr 15 2018 7.3.0 -> 7
在5、7和v1的每个目录中，都存在一个名为iostream的文件。
在/usr/include/x86_64-linux-gnu中也存在一个c++目录，它看起来和这个目录完全一样（有5、7、5.5.0和7.3.0目录）。
在/usr/include中也存在一个c++目录，它看起来和上面的两个目录完全一样
我不知道我的开发环境是如何变得如此混乱的，但在这一点上，我只想知道如何修复它，使Clang++能够成功地找到这9个iostream的实例之一，而不是抛出一个错误，说它不存在。我是否需要添加一个环境变量来告诉Clang去哪里找？我是否需要传递一个命令行参数来告诉Clang进行递归搜索？
Update (1)
When I try building with libc++ I get the following error:
$> clang++ -stdlib=libc++ main.cpp
/usr/bin/ld: cannot find -lc++abi
clang: error: linker command failed with exit code 1 (use -v to see invocation)
当我尝试用手动覆盖的include路径进行构建时，我得到了以下错误。
$> clang++ -isystem /usr/include/c++/7 -isystem /usr/include/x86_64-linux-gnu/c++/7 main.cpp
/usr/bin/ld: cannot find -lstdc++
clang: error: linker command failed with exit code 1 (use -v to see invocation)
当我尝试这两种方法时，我得到了以下（难以置信的大）错误。
$> clang++ -isystem /usr/include/c++/7 -isystem /usr/include/x86_64-linux-gnu/c++/7 -stdlib=libc++ main.cpp
In file included from main.cpp:1:
In file included from /usr/include/c++/7/iostream:39:
In file included from /usr/include/c++/7/ostream:38:
In file included from /usr/include/c++/7/ios:42:
In file included from /usr/include/c++/7/bits/ios_base.h:41:
In file included from /usr/include/c++/7/bits/locale_classes.h:40:
In file included from /usr/include/c++/7/string:52:
In file included from /usr/include/c++/7/bits/basic_string.h:6352:
In file included from /usr/include/c++/7/ext/string_conversions.h:41:
In file included from /usr/include/c++/7/cstdlib:77:
/usr/include/c++/7/bits/std_abs.h:56:3: error: declaration conflicts with target of using declaration already in scope
abs(long __i) { return __builtin_labs(__i); }
/usr/include/c++/v1/stdlib.h:111:44: note: target of using declaration
inline _LIBCPP_INLINE_VISIBILITY long abs( long __x) _NOEXCEPT {return labs(__x);}
/usr/include/c++/7/bits/std_abs.h:52:11: note: using declaration
using ::abs;
/usr/include/c++/7/bits/std_abs.h:61:3: error: declaration conflicts with target of using declaration already in scope
abs(long long __x) { return __builtin_llabs (__x); }
/usr/include/c++/v1/stdlib.h:113:44: note: target of using declaration
inline _LIBCPP_INLINE_VISIBILITY long long abs(long long __x) _NOEXCEPT {return llabs(__x);}
/usr/include/c++/7/bits/std_abs.h:52:11: note: using declaration
using ::abs;
In file included from main.cpp:1:
In file included from /usr/include/c++/7/iostream:39:
In file included from /usr/include/c++/7/ostream:38:
In file included from /usr/include/c++/7/ios:42:
In file included from /usr/include/c++/7/bits/ios_base.h:41:
In file included from /usr/include/c++/7/bits/locale_classes.h:40:
In file included from /usr/include/c++/7/string:52:
In file included from /usr/include/c++/7/bits/basic_string.h:6352:
In file included from /usr/include/c++/7/ext/string_conversions.h:41:
/usr/include/c++/7/cstdlib:177:3: error: declaration conflicts with target of using declaration already in scope
div(long __i, long __j) { return ldiv(__i, __j); }
/usr/include/c++/v1/stdlib.h:116:42: note: target of using declaration
inline _LIBCPP_INLINE_VISIBILITY ldiv_t div( long __x, long __y) _NOEXCEPT {return ldiv(__x, __y);}
/usr/include/c++/7/cstdlib:145:11: note: using declaration
using ::div;
作为一个提醒。我只是在尝试编译《Hello, World》。
我还试着用以下命令卸载并重新安装了Clang。
$> sudo apt-get purge --auto-remove clang
$> sudo apt-get update
$> sudo apt-get install clang
这没有任何效果。我运行的是Ubuntu 18.04，我不知道哪里出了问题，也不知道该从哪里开始修复它。我的构建环境一团糟。
如果可能的话，我想让Clang工作，而不是退回到使用G++，因为我的IDE似乎自动检测Clang并使用它进行语法检查。这意味着我写的每一个C++程序在第一行都有一个致命的错误（"iostream not found"），而文件的其他部分则没有被检查，因为第一行是一个致命的错误。
Update (2)
我试着从Ubuntu apt软件库中再安装一些软件包，但没有成功。
$> sudo apt-get install libc++1 libc++1-9 libc++abi1 libc++abi1-9 llvm-9 llvm-9-dev
$> clang++ -isystem /usr/include/c++/7 -isystem /usr/include/x86_64-linux-gnu/c++/7 -stdlib=libc++ main.cpp
/usr/bin/ld: cannot find -lc++
clang: error: linker command failed with exit code 1 (use -v to see invocation)
我也试过sudo apt-get install lc++1，但发现这是一个完全不相干包。
Update (3)
我又花了几个小时试图解决这个问题，从apt和源代码中安装多个软件包，尝试不同版本的各种工具，从其他来源手动复制库，甚至跳上Clang IRC，直接与几个非常有经验的开发人员交谈。
没有人能够弄清楚我的笔记本电脑出了什么问题，我所做的一切都无法让它工作。
不幸的是，再过两周我就没有这台笔记本了，所以我很可能需要把这个问题关闭为 "无法重现"--因为一旦这台笔记本不在了，我就没有办法重现这个破碎的开发环境了。
