[libcxx] Avoid infinite recursion in create_directories, if the root directory doesn't exist

[mstorsjo]

Summary

[curdeius]

LGTM.

libcxx/src/filesystem/operations.cpp

     return err.report(errc::file_exists);
 
   const path parent = p.parent_path();
   if (!parent.empty()) {
     const file_status parent_st = status(parent, m_ec);
     if (not status_known(parent_st))
       return err.report(m_ec);
     if (not exists(parent_st)) {
+      if (parent == p)
+        return err.report(errc::invalid_argument);
       __create_directories(parent, ec);
       if (ec && *ec) {
         return false;
       }
     } else if (not is_directory(parent_st))
       return err.report(errc::not_a_directory);
   }
   return __create_directory(p, ec);

libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.create_directories/create_directories.pass.cpp

TEST_CASE(dest_final_part_is_file)
     std::error_code ec = GetTestEC();
     TEST_CHECK(fs::create_directories(dir, ec) == false);
     TEST_CHECK(ec);
     TEST_CHECK(ErrorIs(ec, std::errc::not_a_directory));
     TEST_CHECK(is_regular_file(file));
     TEST_CHECK(!exists(dir));
 }
 
+#ifdef _WIN32
+TEST_CASE(nonexistent_root)
+{
+    std::error_code ec = GetTestEC();
+    // If Q:\ doesn't exist, create_directories would try to recurse upwards
+    // to parent_path() until it finds a directory that does exist. As the
+    // whole path is the root name, parent_path() returns itself, and it
+    // would recurse indefinitely, unless the recursion is broken.
+    if (!exists("Q:\\"))
+       TEST_CHECK(fs::create_directories("Q:\\", ec) == false);
+    TEST_CHECK(fs::create_directories("\\\\nonexistentserver", ec) == false);
+}
+#endif
+
 TEST_SUITE_END()