Display a useful error when using the old-style API

Override the copy constructor of `combo_t`, so that we can display an error when
initializing using the old-style API.

Fixes #8.

Signed-off-by: Gergely Nagy <algernon@keyboard.io>

src/Kaleidoscope/MagicCombo.h

@@ -31,6 +31,13 @@
   }                                                                       \
   }
 
+#define _MAGICCOMBO_API_CHANGE                                            \
+  "The MagicCombo API changed in an incompatible way, you will need to\n" \
+  "upgrade.\n"                                                            \
+  "\n"                                                                    \
+  "Please see the `UPGRADING.md` document shipped with the source:\n"     \
+  " https://github.com/keyboardio/Kaleidoscope-MagicCombo/blob/master/UPGRADING.md"
+
 namespace kaleidoscope {
 
 class MagicCombo : public kaleidoscope::Plugin {
@@ -43,8 +50,15 @@ class MagicCombo : public kaleidoscope::Plugin {
   typedef struct combo_t {
     uint32_t left_hand, right_hand;
 
-   private:
-    combo_t(byte left, byte right) {}
+    template <typename T>
+    struct always_false {
+      enum { value = false };
+    };
+
+    template <typename T>
+    combo_t(T l, T r) {
+      static_assert(always_false<T>::value, _DEPRECATE(_MAGICCOMBO_API_CHANGE));
+    }
   } combo_t;
 
   MagicCombo(void) {}