Fixed compile-error for parameter

(need to do the same for other value types, too)

include/sqlpp11/integral.h

@@ -42,6 +42,8 @@ namespace sqlpp
 		// integral value type
 		struct integral
 		{
+#warning why do I need base_value_type? Would it be sufficient to use _value_type instead?
+#warning what is the difference between _is_value and _is_expression?
 			using _base_value_type = integral;
 			using _is_numeric = std::true_type;
 			using _is_integral = std::true_type;
@@ -49,6 +51,8 @@ namespace sqlpp
 			using _is_expression = std::true_type;
 			using _cpp_value_type = int64_t;
 
+			using _value_type = integral;
+			
 			struct _parameter_t
 			{
 				using _value_type = integral;

include/sqlpp11/parameter.h

@@ -28,6 +28,7 @@
 #define SQLPP_PARAMETER_H
 
 #include <sqlpp11/type_traits.h>
+#include <sqlpp11/alias_provider.h>
 #include <sqlpp11/detail/type_set.h>
 
 namespace sqlpp
@@ -81,7 +82,7 @@ namespace sqlpp
 		auto parameter(const ValueType&, const AliasProvider&)
 		-> parameter_t<ValueType, AliasProvider>
 		{
-			static_assert(is_expression_t<ValueType>::value, "first argument is not an expression");
+			static_assert(is_value_t<ValueType>::value, "first argument is not a value type");
 			static_assert(is_alias_provider_t<AliasProvider>::value, "second argument is not an alias provider");
 			return {};
 		}