":" should be an illegal file path on macOS. However, observe the following code:
  auto file_path = base::FilePath(FILE_PATH_LITERAL(":"));
  constexpr int file_flags = base::File::FLAG_CREATE | base::File::FLAG_WRITE |
                             base::File::FLAG_EXCLUSIVE_WRITE;
  base::File file(file_path, file_flags);
  if (!file.IsValid() || !file.created()) {
    // Expected
  } else {
    // Unexpected
  }
Unexpectedly, the else-statement is entered.