Compile with both, MSVC 2015 and 2017

Make MSVC 2017 accept the code as well

MSVC still has problems with variadic templates...

Adjust README, do not do debug build (reduce load for appveyor)

.appveyor.yml

@@ -1,10 +1,12 @@
-os: Visual Studio 2015
+os:
+  - Visual Studio 2015
+  - Visual Studio 2017
 
 platform:
   - x64
 
 configuration:
-  - Debug
+  #- Debug
   - Release
 
 matrix:

README.md

@@ -17,7 +17,7 @@ See [Changes](ChangeLog.md)
 Status:
 -------
 
-Branch / Compiler | clang-3.4,  gcc-4.9, Xcode-7   |  MSVC 2015  | Test Coverage
+Branch / Compiler | clang-3.4,  gcc-4.9, Xcode-7   |  MSVC 2015/2017  | Test Coverage
 ------------------| -------------------------------|-------------|---------------
 master | [![Build Status](https://travis-ci.org/rbock/sqlpp11.svg?branch=master)](https://travis-ci.org/rbock/sqlpp11?branch=master) | [![Build status](https://ci.appveyor.com/api/projects/status/eid7mwqgavo0h61h/branch/master?svg=true)](https://ci.appveyor.com/project/rbock/sqlpp11/branch/master) | [![Coverage Status](https://coveralls.io/repos/rbock/sqlpp11/badge.svg?branch=master)](https://coveralls.io/r/rbock/sqlpp11?branch=master)
 develop | [![Build Status](https://travis-ci.org/rbock/sqlpp11.svg?branch=develop)](https://travis-ci.org/rbock/sqlpp11?branch=develop) | [![Build status](https://ci.appveyor.com/api/projects/status/eid7mwqgavo0h61h/branch/develop?svg=true)](https://ci.appveyor.com/project/rbock/sqlpp11/branch/develop) | [![Coverage Status](https://coveralls.io/repos/rbock/sqlpp11/badge.svg?branch=develop)](https://coveralls.io/r/rbock/sqlpp11?branch=develop)

include/sqlpp11/having.h

@@ -181,6 +181,12 @@ namespace sqlpp
       static_check_t<not std::is_same<Database, void>::value, assert_having_dynamic_statement_dynamic_t>,
       check_having_t<Expression>>;
 
+  template <typename... Exprs>
+  constexpr auto are_all_parameters_expressions() -> bool
+  {
+    return logic::all_t<is_expression_t<Exprs>::value...>::value;
+  }
+
   // NO HAVING YET
   struct no_having_t
   {
@@ -237,7 +243,7 @@ namespace sqlpp
       //	  template <typename... T>
       //	  using _check = logic::all_t<is_expression_t<T>::value...>;
       template <typename... T>
-      struct _check : logic::all_t<is_expression_t<T>::value...>
+      struct _check : std::integral_constant<bool, are_all_parameters_expressions<T...>()>
       {
       };
 

include/sqlpp11/statement.h

@@ -54,6 +54,12 @@ namespace sqlpp
 
   namespace detail
   {
+    template <typename... Policies>
+    constexpr auto is_any_policy_missing() -> bool
+    {
+      return logic::any_t<is_missing_t<Policies>::value...>::value;
+    }
+
     template <typename Db = void, typename... Policies>
     struct statement_policies_t
     {
@@ -122,11 +128,10 @@ namespace sqlpp
       }
 
       using _value_type =
-          typename std::conditional<logic::none_t<is_missing_t<Policies>::value...>::value,
+          typename std::conditional<is_any_policy_missing<Policies...>(),
-                                    value_type_of<_result_type_provider>,
+                                    no_value_t,  // if a required statement part is missing (e.g. columns in a select),
-                                    no_value_t  // if a required statement part is missing (e.g. columns in a select),
                                                  // then the statement cannot be used as a value
-                                    >::type;
+                                    value_type_of<_result_type_provider>>::type;
 
       using _traits =
           make_traits<_value_type, tag_if<tag::is_expression, not std::is_same<_value_type, no_value_t>::value>>;