Fix derived → base conversions

Author: jfirebaugh

include/jni/object.hpp

@@ -54,14 +54,20 @@ namespace jni
     template < class Tag, class Enable = void >
     struct SuperObject;
 
+    template < class Tag, class = int >
+    struct HasSuperTag : std::false_type {};
+
+    template < class Tag >
+    struct HasSuperTag<Tag, decltype(std::declval<typename Tag::SuperTag>(), 0)> : std::true_type {};
+
     template < class Tag >
-    struct SuperObject<Tag, typename Tag::SuperTag>
+    struct SuperObject<Tag, std::enable_if_t<HasSuperTag<Tag>::value>>
        {
         using Type = Object<typename Tag::SuperTag>;
        };
 
     template < class Tag >
-    struct SuperObject<Tag>
+    struct SuperObject<Tag, std::enable_if_t<!HasSuperTag<Tag>::value>>
        {
         using Type = Object<ObjectTag>;
        };

include/jni/tagging.hpp

@@ -65,7 +65,7 @@ namespace jni
                  deleter(std::move(other.get_deleter())) {}
 
             template < class U >
-            Tagged(Tagged<U, D>&& other)
+            Tagged(Tagged<U, D>&& other, std::enable_if_t<std::is_convertible<const U&, const T&>::value>* = nullptr)
                : T(other.release()),
                  deleter(std::move(other.get_deleter())) {}
 

test/high_level.cpp

@@ -23,7 +23,10 @@ namespace
        };
 
     struct Base {};
-    struct Derived : public Base {};
+    struct Derived
+       {
+        using SuperTag = Base;
+       };
    }
 
 template < char... Cs >