From 1c95406ceddd8271eec870cfb4bea15217d0a3f3 Mon Sep 17 00:00:00 2001
From: rbock <rbock@eudoxos.de>
Date: Wed, 26 Mar 2014 21:01:16 +0100
Subject: [PATCH] Improved error messages for incorrect parameters.

---
 include/sqlpp11/alias_provider.h | 14 ++++++++++++++
 include/sqlpp11/parameter.h      |  4 +++-
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/include/sqlpp11/alias_provider.h b/include/sqlpp11/alias_provider.h
index d63ce097..a0cd3789 100644
--- a/include/sqlpp11/alias_provider.h
+++ b/include/sqlpp11/alias_provider.h
@@ -27,6 +27,8 @@
 #ifndef SQLPP_ALIAS_PROVIDER_H
 #define SQLPP_ALIAS_PROVIDER_H
 
+#include <type_traits>
+
 #define SQLPP_ALIAS_PROVIDER(name) \
 	struct name##_t\
 	{\
@@ -46,6 +48,18 @@
 
 namespace sqlpp
 {
+	template<typename T, typename Enable = void>
+		struct is_alias_provider_t
+		{
+			static constexpr bool value = false;
+		};
+
+	template<typename T>
+		struct is_alias_provider_t<T, typename std::enable_if<std::is_class<typename T::_name_t::template _member_t<int>>::value, void>::type>
+		{
+			static constexpr bool value = true;
+		};
+
 	namespace alias
 	{
 		SQLPP_ALIAS_PROVIDER(a);
diff --git a/include/sqlpp11/parameter.h b/include/sqlpp11/parameter.h
index 8029ad02..c4f183fc 100644
--- a/include/sqlpp11/parameter.h
+++ b/include/sqlpp11/parameter.h
@@ -35,7 +35,6 @@ namespace sqlpp
 	template<typename ValueType, typename NameType>
 	struct parameter_t: public ValueType::template expression_operators<parameter_t<ValueType, NameType>>
 	{
-#warning need to check that Value Type is an SQL value type!
 		struct _value_type: public ValueType
 		{
 			using _is_expression = std::true_type;
@@ -74,6 +73,7 @@ namespace sqlpp
 		auto parameter(const NamedExpr&)
 		-> parameter_t<typename NamedExpr::_value_type, NamedExpr>
 		{
+			static_assert(is_named_expression_t<NamedExpr>::value, "not a named expression");
 			return {};
 		}
 
@@ -81,6 +81,8 @@ 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_alias_provider_t<AliasProvider>::value, "second argument is not an alias provider");
 			return {};
 		}