From 3efae18e081d2e2634a54d5da573298c00e1a3c9 Mon Sep 17 00:00:00 2001
From: rbock <rbock@eudoxos.de>
Date: Wed, 23 Dec 2015 14:45:40 +0100
Subject: [PATCH] Friendlier error messages for some case-expressions

---
 include/sqlpp11/case.h        | 2 +-
 include/sqlpp11/no_value.h    | 3 ++-
 include/sqlpp11/type_traits.h | 3 ++-
 3 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/include/sqlpp11/case.h b/include/sqlpp11/case.h
index c16151e1..11877fca 100644
--- a/include/sqlpp11/case.h
+++ b/include/sqlpp11/case.h
@@ -55,7 +55,7 @@ namespace sqlpp
   struct case_t
       : public expression_operators<
             case_t<When, Then, Else>,
-            typename std::conditional<is_sql_null_t<Then>::value, value_type_of<When>, value_type_of<Then>>::type>,
+            typename std::conditional<is_sql_null_t<Then>::value, value_type_of<Else>, value_type_of<Then>>::type>,
         public alias_operators<case_t<When, Then, Else>>
   {
     using _traits = make_traits<value_type_of<Then>, tag::is_expression>;
diff --git a/include/sqlpp11/no_value.h b/include/sqlpp11/no_value.h
index 68c842dc..15703d0e 100644
--- a/include/sqlpp11/no_value.h
+++ b/include/sqlpp11/no_value.h
@@ -35,7 +35,8 @@ namespace sqlpp
 {
   struct no_value_t
   {
-    using _traits = make_traits<void>;
+    using _traits = make_traits<no_value_t>;
+    using _cpp_value_type = void;
   };
 
   template <typename Base>
diff --git a/include/sqlpp11/type_traits.h b/include/sqlpp11/type_traits.h
index 83f49e16..8c978de6 100644
--- a/include/sqlpp11/type_traits.h
+++ b/include/sqlpp11/type_traits.h
@@ -38,12 +38,13 @@
 
 namespace sqlpp
 {
+  struct no_value_t;
   namespace detail
   {
     template <typename T, typename Enable = void>
     struct value_type_of_impl
     {
-      static_assert(wrong_t<value_type_of_impl>::value, "Attempting to obtain value type from type without value_type");
+      using type = no_value_t;
     };
 
     template <typename T>