Maybe easier to grok for MSVC

Author: rbock

include/sqlpp11/union.h

@@ -215,7 +215,8 @@ 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>;
-        static_assert(lhs_result_row_t::is_compatible(detail::type_vector<rhs_result_row_t>{}),
+        constexpr auto vec_rhs = detail::type_vector<rhs_result_row_t>{};
+        static_assert(lhs_result_row_t::is_compatible(vec_rhs),
                       "both arguments in a union have to have the same result columns (type and name)");
         static_assert(
             is_static_result_row_t<lhs_result_row_t>::value && is_static_result_row_t<rhs_result_row_t>::value,
@@ -236,7 +237,8 @@ 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>;
-        static_assert(lhs_result_row_t::is_compatible(detail::type_vector<rhs_result_row_t>{}),
+        constexpr auto vec_rhs = detail::type_vector<rhs_result_row_t>{};
+        static_assert(lhs_result_row_t::is_compatible(vec_rhs),
                       "both arguments in a union have to have the same result columns (type and name)");
         static_assert(
             is_static_result_row_t<lhs_result_row_t>::value && is_static_result_row_t<rhs_result_row_t>::value,