Clang 模块与 std <iterator> 和 <boost/move/iterator.hpp> 的交互

Question

（有关我正在使用的特定版本的 Boost 和 Clang 的信息，请参阅问题的结尾）

使用新的实验性 -fmodules 功能从 master/HEAD 在 Clang 中编译，使用如下所示的命令行选项编译以下文件时出现构建错误：

#include <iterator>
#include <boost/move/iterator.hpp>

编译命令和错误：

anhall@leviathan: <path-to-clang-install-from-master>/bin/clang++ -o file.o -c file.cpp --std=c++1z -stdlib=libc++ -fmodules

In file included from file.cpp:2:
In file included from /usr/local/include/boost/move/iterator.hpp:27:
/usr/local/include/boost/move/detail/iterator_traits.hpp:60:17: error: reference to 'random_access_iterator_tag' is ambiguous
   typedef std::random_access_iterator_tag   iterator_category;
                ^
/Users/anhall/impersonal/code/llvm-reflexpr/install/bin/../include/c++/v1/iterator:438:30: note: candidate found by name lookup is 'std::__1::random_access_iterator_tag'
struct _LIBCPP_TYPE_VIS_ONLY random_access_iterator_tag : public bidirectional_iterator_tag {};
                             ^
/usr/local/include/boost/move/detail/iterator_traits.hpp:34:8: note: candidate found by name lookup is 'std::random_access_iterator_tag'
struct random_access_iterator_tag;
       ^
/usr/local/include/boost/move/detail/iterator_traits.hpp:71:17: error: reference to 'random_access_iterator_tag' is ambiguous
   typedef std::random_access_iterator_tag   iterator_category;
                ^
/Users/anhall/impersonal/code/llvm-reflexpr/install/bin/../include/c++/v1/iterator:438:30: note: candidate found by name lookup is 'std::__1::random_access_iterator_tag'
struct _LIBCPP_TYPE_VIS_ONLY random_access_iterator_tag : public bidirectional_iterator_tag {};
                             ^
/usr/local/include/boost/move/detail/iterator_traits.hpp:34:8: note: candidate found by name lookup is 'std::random_access_iterator_tag'
struct random_access_iterator_tag;
       ^
In file included from file.cpp:2:
/usr/local/include/boost/move/iterator.hpp:196:17: error: reference to 'output_iterator_tag' is ambiguous
   typedef std::output_iterator_tag    iterator_category;
                ^
/Users/anhall/impersonal/code/llvm-reflexpr/install/bin/../include/c++/v1/iterator:435:30: note: candidate found by name lookup is 'std::__1::output_iterator_tag'
struct _LIBCPP_TYPE_VIS_ONLY output_iterator_tag {};
                             ^
/usr/local/include/boost/move/detail/iterator_traits.hpp:35:8: note: candidate found by name lookup is 'std::output_iterator_tag'
struct output_iterator_tag;
       ^
In file included from file.cpp:2:
/usr/local/include/boost/move/iterator.hpp:238:17: error: reference to 'output_iterator_tag' is ambiguous
   typedef std::output_iterator_tag    iterator_category;
                ^
/Users/anhall/impersonal/code/llvm-reflexpr/install/bin/../include/c++/v1/iterator:435:30: note: candidate found by name lookup is 'std::__1::output_iterator_tag'
struct _LIBCPP_TYPE_VIS_ONLY output_iterator_tag {};
                             ^
/usr/local/include/boost/move/detail/iterator_traits.hpp:35:8: note: candidate found by name lookup is 'std::output_iterator_tag'
struct output_iterator_tag;
       ^
In file included from file.cpp:2:
/usr/local/include/boost/move/iterator.hpp:278:17: error: reference to 'output_iterator_tag' is ambiguous
   typedef std::output_iterator_tag    iterator_category;
                ^
/Users/anhall/impersonal/code/llvm-reflexpr/install/bin/../include/c++/v1/iterator:435:30: note: candidate found by name lookup is 'std::__1::output_iterator_tag'
struct _LIBCPP_TYPE_VIS_ONLY output_iterator_tag {};
                             ^
/usr/local/include/boost/move/detail/iterator_traits.hpp:35:8: note: candidate found by name lookup is 'std::output_iterator_tag'
struct output_iterator_tag;
       ^
5 errors generated.

如果我删除 #include <iterator> 或将其移到 #include <boost/move/iterator.hpp> 之后，错误就会消失。

问题：这仅仅是 Boost 库（显然是 Boost Move）没有为具有 Clang 模块映射文件的模块准备的副产品吗？这可能是 Clang 现在为 libc++ 实现的模块映射文件的错误，甚至是模块实现本身的错误？

有趣的是，我可以通过注释掉 boost/move/detail/iterator_traits.hpp 的第 28-28 行来消除错误：

// #include <boost/move/detail/std_ns_begin.hpp>
// BOOST_MOVE_STD_NS_BEG
//
// struct input_iterator_tag;
// struct forward_iterator_tag;
// struct bidirectional_iterator_tag;
// struct random_access_iterator_tag;
// struct output_iterator_tag;
//
// BOOST_MOVE_STD_NS_END
// #include <boost/move/detail/std_ns_end.hpp>

正在使用的版本

提升 1.61

截至 2016 年 12 月 7 日，Clang 来自 LLVM 的 github 镜像中的 master 分支的尖端（技术上它来自 LLVM 的一个分支，但它仅在与 LLVM 自己的 master 分支相同的 master 分支上）：

clang version 4.0.0 (https://github.com/matus-chochlik/clang.git b9cb1c8a1ebf52695372de12c7b04c8ef1bd8b4e) (https://github.com/llvm-mirror/llvm.git b60c7b1f61eabbe971d08568adb790a7cfc6a403)
Target: x86_64-apple-darwin15.6.0
Thread model: posix
InstalledDir: /Users/anhall/impersonal/code/llvm-reflexpr/install/bin

Answer 1

我认为这是由 libc++ 的 module.modulemap 问题引起的，我昨天在 r289028 中修复了该问题。此错误导致 _LIBCPP_VERSION 宏在启用模块时无法由 libc++ 导出。

查看boost/move/detail/iterator_traits.hpp 后，似乎如果_LIBCPP_VERSION 未定义，它将错过配置并最终声明完全不同的迭代器标签定义。 （具体来说，它将直接在命名空间std 中定义它们，而不是在 libc++ 的版本控制命名空间中）。

如果您重新构建 LLVM 和 libc++，它应该解决您的问题。如果没有，请随时file a bug。

PS。看来我只是one step ahead。

Answer 2

目前我只能说 boost 1.59.1，但它显然也假设了 libc++ 只能在使用 clang 构建时遇到：

#if defined(__clang__) && defined(_LIBCPP_VERSION)
// ...
#else
// ...
#endif

即使在 Mac OS X 10.9 上，它也会引发（几乎）完全相同的错误，它的 libc++ 早于引入导致本线程中讨论的问题的 libc++ 回归。