Add copy assignment operator for Error
diff --git a/include/cxx.h b/include/cxx.h
index adbc8e6..d43854c 100644
--- a/include/cxx.h
+++ b/include/cxx.h
@@ -257,6 +257,9 @@
   Error(const Error &);
   Error(Error &&) noexcept;
   ~Error() noexcept;
+
+  Error &operator=(const Error &);
+
   const char *what() const noexcept override;
 
 private: