Add trap and unreachable

* Use both functions to exit on invalid behaviour

include/continuable/detail/base.hpp

@@ -387,9 +387,9 @@ struct error_callback<hints::signature_hint_tag<Args...>, Callback, Executor,
   }
 };
 
-/// Workaround for GCC bug:
+/// Once this was a workaround for GCC bug:
 /// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64095
-struct empty_callback {
+struct final_callback {
   template <typename... Args>
   void operator()(Args... /*args*/) {
   }
@@ -398,7 +398,14 @@ struct empty_callback {
   void set_value(Args... /*args*/) {
   }
 
-  void set_exception(types::error_type /*error*/) {
+  void set_exception(types::error_type error) {
+    (void)error;
+#ifndef CONTINUABLE_WITH_UNHANDLED_ERRORS
+    // There were unhandled errors inside the asynchronous call chain!
+    // Define `CONTINUABLE_WITH_UNHANDLED_ERRORS` in order
+    // to ignore unhandled errors!"
+    util::trap();
+#endif // CONTINUABLE_WITH_UNHANDLED_ERRORS
   }
 };
 } // namespace callbacks
@@ -518,7 +525,7 @@ auto chain_error_handler(Continuation&& continuation, Callback&& callback,
 template <typename Continuation>
 void finalize_continuation(Continuation&& continuation) {
   attorney::invoke_continuation(std::forward<Continuation>(continuation),
-                                callbacks::empty_callback{});
+                                callbacks::final_callback{});
 }
 
 /// Workaround for GCC bug:

include/continuable/detail/hints.hpp

@@ -34,6 +34,7 @@
 #include <type_traits>
 
 #include <continuable/detail/api.hpp>
+#include <continuable/detail/traits.hpp>
 #include <continuable/detail/util.hpp>
 
 namespace cti {

include/continuable/detail/transforms.hpp

@@ -37,6 +37,7 @@
 #include <continuable/detail/features.hpp>
 #include <continuable/detail/hints.hpp>
 #include <continuable/detail/types.hpp>
+#include <continuable/detail/util.hpp>
 
 namespace cti {
 namespace detail {
@@ -92,13 +93,19 @@ public:
   void operator()(Args... args) {
     this->resolve(promise_, std::move(args)...);
   }
-#if !defined(CONTINUABLE_WITH_CUSTOM_ERROR_TYPE) &&                            \
-    !defined(CONTINUABLE_WITH_NO_EXCEPTIONS)
+
   /// Resolves the promise through the exception
   void operator()(types::dispatch_error_tag, types::error_type error) {
+#if !defined(CONTINUABLE_WITH_CUSTOM_ERROR_TYPE) &&                            \
+    !defined(CONTINUABLE_WITH_NO_EXCEPTIONS)
     promise_.set_exception(error);
-  }
+#else
+    // Can't forward a std::error_condition or custom error type
+    // to a std::promise. Handle the error first in order
+    // to prevent this trap!
+    util::trap();
 #endif
+  }
 
   /// Returns the future from the promise
   auto get_future() {

include/continuable/detail/util.hpp

@@ -241,6 +241,27 @@ private:
   /// Is true when the automatic invocation on destruction is disabled
   bool frozen_ : 1;
 };
+
+/// Hint for the compiler that this point should be unreachable
+[[noreturn]] inline void unreachable() {
+#ifdef _MSC_VER
+  __assume(false);
+#elif __has_builtin(__builtin_unreachable)
+  __builtin_unreachable();
+#endif
+}
+
+/// Causes the application to exit abnormally because we are
+/// in an invalid state.
+[[noreturn]] inline void trap() {
+#ifdef _MSC_VER
+  __debugbreak();
+#elif __has_builtin(__builtin_trap)
+  __builtin_trap();
+#else
+  *(volatile int*)0 = 0;
+#endif
+}
 } // namespace util
 } // namespace detail
 } // namespace cti