Can't use UDL of `std::string` with `import std;`

[JohelEGP]

With https://libcxx.llvm.org/Modules.html#using-in-external-projects,
the following program fails:

export module main;
import std;
using namespace std::string_literals;

main.cpp:3:22: error: missing '#include <string>'; 'string_literals' must be declared before it is used
    3 | using namespace std::string_literals;
      |                      ^
/home/johel/Documents/C++/Forks/llvm/libcxx-std-modules/include/c++/v1/string:4387:20: note: declaration here is not visible
 4387 |   inline namespace string_literals
      |                    ^
1 error generated.

[llvmbot]

@llvm/issue-subscribers-clang-modules

Author: Johel Ernesto Guerrero Peña (JohelEGP)

With <https://libcxx.llvm.org/Modules.html#using-in-external-projects>, the following program fails: ```C++ export module main; import std; using namespace std::string_literals; ``` ``` main.cpp:3:22: error: missing '#include <string>'; 'string_literals' must be declared before it is used 3 | using namespace std::string_literals; | ^ /home/johel/Documents/C++/Forks/llvm/libcxx-std-modules/include/c++/v1/string:4387:20: note: declaration here is not visible 4387 | inline namespace string_literals | ^ 1 error generated. ```

[ChuanqiXu9]

I am wondering if this is a libc++'s issue. I've checked that in modules/std/string.inc, only the namespace std::literals and std::literals::string_literals are exported and std::string_literals are not mentioned. @mordante

[JohelEGP]

using namespace std::literals::string_literals; fails equally.

[ChuanqiXu9]

using namespace std::literals::string_literals; fails equally.

Got it. I'll try to take a look.

[ChuanqiXu9]

After I took more experiment in the library level, I feel this is a library issue more.

First, it is not a problem to using namespace std::literals;. Since there are multiple use inline namespace literals inside *.inc under build/_deps/std-src/std.

Then for std::literals::string_literals, it is guarded by #if directives in string.inc:

// TODO MODULES is this a bug?
#if _LIBCPP_STD_VER >= 23
  using std::operator""s;
#else
  inline namespace literals {
    inline namespace string_literals {
      // [basic.string.literals], suffix for basic_string literals
      using std::literals::string_literals::operator""s;
    } // namespace string_literals
  }   // namespace literals
#endif

It looks like the #else branch is not taken since I added an #error directive but the std module still compiles.

Finally, I add the namespaces manually in string.inc:

inline namespace literals {
    inline namespace string_literals {}
  }

Then using std::literals::string_literals; compiles too.

For std::string_literals, since it doesn't appear in any place under build/_deps/std-src/std, so I believe it is not exported.