From e39444b0f70e613b0008fdc760d773538a50d7f2 Mon Sep 17 00:00:00 2001
From: rbock <rbock@eudoxos.de>
Date: Mon, 19 May 2014 06:42:45 +0200
Subject: [PATCH] Added tags missing and return_value

missing indicates that a statement part has yet to be added (e.g.
columns in a select).

return_value indicates that a statement part can be used as a return
value.
---
 include/sqlpp11/detail/type_set.h           | 3 +++
 include/sqlpp11/select.h                    | 4 ++--
 include/sqlpp11/type_traits.h               | 2 ++
 include/sqlpp11/vendor/select_column_list.h | 6 +++---
 4 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/include/sqlpp11/detail/type_set.h b/include/sqlpp11/detail/type_set.h
index 64a2b005..ecc51a23 100644
--- a/include/sqlpp11/detail/type_set.h
+++ b/include/sqlpp11/detail/type_set.h
@@ -157,6 +157,9 @@ namespace sqlpp
 				using type = typename make_type_set_if<Predicate, Rest...>::type::template insert_if<Predicate, T>::type;
 			};
 
+		template<template<typename> class Predicate, typename... T>
+			using make_type_set_if_t = typename make_type_set_if<Predicate, T...>::type;
+
 		template<template<typename> class Predicate, typename... T>
 			struct make_type_set_if_not
 			{
diff --git a/include/sqlpp11/select.h b/include/sqlpp11/select.h
index da752636..2f5dc271 100644
--- a/include/sqlpp11/select.h
+++ b/include/sqlpp11/select.h
@@ -145,9 +145,9 @@ namespace sqlpp
 					>::type;
 
 				using _value_type = typename std::conditional<
-					is_select_column_list_t<_column_list_t>::value and is_subset_of<required_tables_of<_column_list_t>, provided_tables_of<_from_t>>::value,
+					detail::make_type_set_if_t<is_missing_t, FlagList, ColumnList, From, ExtraTables, Where, GroupBy, Having, OrderBy, Limit, Offset>::size::value == 0,
 					value_type_of<_column_list_t>,
-					no_value_t // If something is selected that requires a table, then we require a from for this to be a value
+					no_value_t // if a required statement part is missing (columns in a select), then the statement cannot be used as a value
 						>::type;
 
 				using _traits = make_traits<_value_type>;
diff --git a/include/sqlpp11/type_traits.h b/include/sqlpp11/type_traits.h
index 66243bc2..a1f231c5 100644
--- a/include/sqlpp11/type_traits.h
+++ b/include/sqlpp11/type_traits.h
@@ -106,6 +106,8 @@ namespace sqlpp
 	SQLPP_IS_COLUMN_TRAIT_GENERATOR(trivial_value_is_null);
 
 	SQLPP_IS_VALUE_TRAIT_GENERATOR(noop);
+	SQLPP_IS_VALUE_TRAIT_GENERATOR(missing);
+	SQLPP_IS_VALUE_TRAIT_GENERATOR(return_value);
 	SQLPP_IS_VALUE_TRAIT_GENERATOR(table);
 	SQLPP_IS_VALUE_TRAIT_GENERATOR(join);
 	SQLPP_IS_VALUE_TRAIT_GENERATOR(pseudo_table);
diff --git a/include/sqlpp11/vendor/select_column_list.h b/include/sqlpp11/vendor/select_column_list.h
index 1f160d16..ce98c227 100644
--- a/include/sqlpp11/vendor/select_column_list.h
+++ b/include/sqlpp11/vendor/select_column_list.h
@@ -47,14 +47,14 @@ namespace sqlpp
 		template<typename... Rest>
 			struct get_first_argument_if_unique
 			{
-				using _traits = make_traits<no_value_t, tag::select_column_list>;
+				using _traits = make_traits<no_value_t, tag::select_column_list, tag::return_value>;
 				struct _name_t {};
 			};
 
 		template<typename T>
 			struct get_first_argument_if_unique<T>
 			{
-				using _traits = make_traits<value_type_of<T>, tag::select_column_list, tag::expression, tag::named_expression>;
+				using _traits = make_traits<value_type_of<T>, tag::select_column_list, tag::return_value, tag::expression, tag::named_expression>;
 				using _name_t = typename T::_name_t;
 			};
 	}
@@ -239,7 +239,7 @@ namespace sqlpp
 	{
 		struct no_select_column_list_t
 		{
-			using _traits = make_traits<no_value_t, ::sqlpp::tag::noop>;
+			using _traits = make_traits<no_value_t, ::sqlpp::tag::noop, ::sqlpp::tag::missing>;
 			using _recursive_traits = make_recursive_traits<>;
 
 			template<typename Db>