Enable implicit construction when the alternative is explicitly constructible from the argument. Fixes #103.

test/result_error_construct.cpp

@@ -146,8 +146,14 @@ int main()
         BOOST_TEST_TRAIT_TRUE((std::is_constructible<result<std::string, X>, int>));
         BOOST_TEST_TRAIT_FALSE((std::is_convertible<int, result<std::string, X>>));
 
-        BOOST_TEST_TRAIT_FALSE((std::is_constructible<result<int, X>, int>));
-        BOOST_TEST_TRAIT_FALSE((std::is_convertible<int, result<int, X>>));
+        // We'd like this to be false due to the ambiguity caused by X having
+        // an explicit constructor taking an int, which should be viable in this
+        // context, but the implicit constructor is enabled, and there's no way to
+        // disallow it
+        //
+        // BOOST_TEST_TRAIT_FALSE((std::is_constructible<result<int, X>, int>));
+
+        BOOST_TEST_TRAIT_TRUE((std::is_convertible<int, result<int, X>>));
     }
 
     {