Replace is_compatible function with class template

MSVC choked on that function (claimed that the number of arguments was
incorrect)

Author: rbock

include/sqlpp11/result_row.h

@@ -186,19 +186,6 @@ namespace sqlpp
     using _impl = detail::result_row_impl<Db, _field_index_sequence, FieldSpecs...>;
     bool _is_valid;
 
-    template <typename D, typename... Fs>
-    static constexpr auto is_compatible(detail::type_vector<result_row_t<D, Fs...>>) ->
-        typename std::enable_if<sizeof...(Fs) == sizeof...(FieldSpecs), bool>::type
-    {
-      return logic::all_t<FieldSpecs::is_compatible(Fs{})...>::value;
-    }
-
-    template <typename D, typename... Fs>
-    static constexpr auto is_compatible(detail::type_vector<result_row_t<D, Fs...>>) ->
-        typename std::enable_if<sizeof...(Fs) != sizeof...(FieldSpecs), bool>::type
-    {
-      return false;
-    }
     result_row_t() : _impl(), _is_valid(false)
     {
     }
@@ -264,6 +251,20 @@ namespace sqlpp
     }
   };
 
+  template <typename Lhs, typename Rhs, typename Enable = void>
+  struct is_result_compatible
+  {
+    static constexpr auto value = false;
+  };
+
+  template <typename LDb, typename... LFields, typename RDb, typename... RFields>
+  struct is_result_compatible<result_row_t<LDb, LFields...>,
+                              result_row_t<RDb, RFields...>,
+                              typename std::enable_if<sizeof...(LFields) == sizeof...(RFields)>::type>
+  {
+    static constexpr auto value = logic::all_t<LFields::is_compatible(RFields{})...>::value;
+  };
+
   template <typename Db, typename... FieldSpecs>
   struct dynamic_result_row_t
       : public detail::result_row_impl<Db, detail::make_field_index_sequence<0, FieldSpecs...>, FieldSpecs...>

include/sqlpp11/union.h

@@ -215,8 +215,7 @@ namespace sqlpp
 
         using lhs_result_row_t = get_result_row_t<derived_statement_t<Policies>>;
         using rhs_result_row_t = get_result_row_t<Rhs>;
-        constexpr auto vec_rhs = detail::type_vector<rhs_result_row_t>{};
-        static_assert(lhs_result_row_t::is_compatible(vec_rhs),
+        static_assert(is_result_compatible<lhs_result_row_t, rhs_result_row_t>::value,
                       "both arguments in a union have to have the same result columns (type and name)");
 
         return _union_impl<void, union_distinct_t>(check_union_t<derived_statement_t<Policies>, Rhs>{}, rhs);
@@ -234,8 +233,7 @@ namespace sqlpp
 
         using lhs_result_row_t = get_result_row_t<derived_statement_t<Policies>>;
         using rhs_result_row_t = get_result_row_t<Rhs>;
-        constexpr auto vec_rhs = detail::type_vector<rhs_result_row_t>{};
-        static_assert(lhs_result_row_t::is_compatible(vec_rhs),
+        static_assert(is_result_compatible<lhs_result_row_t, rhs_result_row_t>::value,
                       "both arguments in a union have to have the same result columns (type and name)");
 
         return _union_impl<void, union_all_t>(check_union_t<derived_statement_t<Policies>, Rhs>{}, rhs);