我写了下面这个简单的C++程序：

#include <iostream>

using namespace std;

int main() {
    cout << "Hello, World" << endl;
    return 0;
}

当我用g编译这个的时候，它工作得很完美，当我试图用Clang编译的时候，我得到了下面的错误：

main.cpp:1:10: fatal error: 'iostream' file not found
#include <iostream>
         ^~~~~~~~~~
1 error generated.

使用-v参数运行，我看到以下内容：

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.

分别查看这些文件夹，我发现在/usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++中（或者更准确地说，在/usr/include/c++中）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

在每个5、7和v1目录中，都存在一个名为iostream的文件
同样在/usr/include/x86_64-linux-gnu中，存在一个c++目录，与此目录完全相同（包含5、7、5.5.0和7.3.0目录）。
此外，/usr/include中还存在一个c++目录，该目录与上面的两个目录完全相同
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++将成功地找到这9个iostream示例之一，而不是抛出一个错误，它不存在.我需要添加一个环境变量来告诉Clang在哪里查找？我需要传递一个命令行参数来告诉Clang递归搜索？

更新（1）

当我尝试使用libc++构建时，我得到了以下错误：

$> 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)

当我同时尝试这两种方法时，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;
          ^

提醒一下我只是在编译Hello，World
我还尝试使用以下命令卸载并重新安装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，我不知道出了什么问题，也不知道从哪里开始修复它。我的构建环境一团糟。
如果可能的话，我希望Clang能正常工作，而不是退回到使用G++，因为我的IDE似乎会自动检测Clang并使用它进行语法检查。这意味着我编写的每个C++程序在第一行都有一个致命错误（“iostream not found”），而文件的其余部分都没有检查，因为第一行是致命错误。

更新（2）

我试着从Ubuntu apt仓库安装了一些软件包，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)

我也尝试了sudo apt-get install lc++1，却发现这是一个***完全无关的***包。

更新（3）

我又花了几个小时试图解决这个问题，从apt和source安装了多个软件包，尝试了各种工具的不同版本，从其他源手动复制到库中，甚至跳到Clang IRC上直接与几个非常有知识的开发人员交谈。
没人能找出我的笔记本电脑出了什么问题，我也没能让它工作。
不幸的是，两周后我就没有这台笔记本电脑了，所以我可能需要将这个问题作为“无法重现”来关闭-因为一旦笔记本电脑没有了，我就没有办法复制这个坏掉的开发环境。