Workaround for MinGW/libstdc++ bug

This is from zaphoyd/websocketpp#479

Author: wch

src/lib/websocketpp/transport/asio/endpoint.hpp

@@ -45,6 +45,69 @@ namespace ws_websocketpp {
 namespace transport {
 namespace asio {
 
+/***
+ * In current versions of libstdc++, the error_constants.h code associated to mingw32
+ * does not define certain standard enum values for `std::errc`. (In C++11 standard,
+ * sections 19.5.2, 19.5.3.) Asio uses these for lib::asio::errc when it is compiled
+ * as a stand-alone library, so because of the libstdc++ defect, code below referring
+ * to lib::asio::errc::operation_canceled fails to compile on mingw.
+ *
+ * This workaround detects the defect using SFINAE and returns 'false' for the check
+ * if operation_canceled is not defined, instead of failing to compile.
+ *
+ * If upstream patches this later by defining those enum values, then the workaround
+ * will stop having any effect.
+ *
+ * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68307
+ */
+namespace _workaround_gcc_libstdcpp_issue_68307_missing_values {
+  /***
+   * Same as std::enable_if, but don't want to introduce dependency on <type_traits>
+   * since that's C++11 only
+   */
+  template<bool B, class T = void>
+  struct enable_if {};
+
+  template<class T>
+  struct enable_if<true, T> { typedef T type; };
+
+  /***
+   * Metafunction to test "operation_canceled" value
+   */
+  template <typename T, typename ENABLE=void>
+  struct op_canceled_helper {
+    template <typename U>
+    static inline bool is_op_canceled(const U & u) { return false; }
+  };
+
+  template <typename T>
+  struct op_canceled_helper<T, enable_if<T::operation_canceled == T::operation_canceled, void> > {
+    template<typename U>
+    static inline bool is_op_canceled(const U & u) { return u == T::operation_canceled; }
+  };
+
+  /***
+   * This function is intended to be a drop-in replacement for
+   *   (asio_ec == lib::asio::errc::operation_canceled)
+   *
+   * except that if lib::asio::errc::operation_canceled does not exist, it returns false,
+   * instead of failing to compile.
+   *
+   * When using boost and not asio standalone, then lib::asio::errc is a namespace, not an enum class.
+   * So the template code will fail to compile and we need to block it from being instantiated, with this
+   * ifdef. When using boost the standard library symbol definitions aren't relevant afaik.
+   */
+#ifdef ASIO_STANDALONE
+  static inline bool is_op_canceled(const lib::asio::error_code & asio_ec) {
+    return op_canceled_helper<lib::asio::errc, void>::is_op_canceled(asio_ec);
+  }
+#else
+  static inline bool is_op_canceled(const lib::asio::error_code & asio_ec) {
+    return asio_ec == lib::asio::errc::operation_canceled;
+  }
+#endif
+} // namespace _workaround
+
 /// Asio based endpoint transport component
 /**
  * transport::asio::endpoint implements an endpoint transport component using
@@ -833,7 +896,8 @@ class endpoint : public config::socket_type {
         m_alog->write(log::alevel::devel, "asio::handle_accept");
 
         if (asio_ec) {
-            if (asio_ec == lib::asio::errc::operation_canceled) {
+            // if (asio_ec == lib::asio::errc::operation_canceled) {
+            if (_workaround_gcc_libstdcpp_issue_68307_missing_values::is_op_canceled(asio_ec)) {
                 ret_ec = make_error_code(ws_websocketpp::error::operation_canceled);
             } else {
                 log_err(log::elevel::info,"asio handle_accept",asio_ec);