shift the property forward decls to forward.h

Summary: reduces folly::Executor dependencies

Reviewed By: yfeldblum

Differential Revision: D14939208

fbshipit-source-id: bc354e838608eb8b25373ed5179e72acef813edd

folly/experimental/pushmi/forwards.h

@@ -27,6 +27,20 @@ namespace pushmi {
 // derive from this for types that need to find operator|() overloads by ADL
 struct pipeorigin {};
 
+// properties types:
+
+template <class T, class = void>
+struct property_traits;
+
+template <class T, class = void>
+struct property_set_traits;
+
+template <class... PropertyN>
+struct property_set;
+
+template <class T, class Target, class = void>
+struct property_set_traits_disable;
+
 // Traits types:
 
 template <class T, class = void>

folly/experimental/pushmi/properties.h

@@ -18,23 +18,14 @@
 #include <folly/experimental/pushmi/traits.h>
 #include <folly/Traits.h>
 
+#include <folly/experimental/pushmi/forwards.h>
+
 namespace folly {
 namespace pushmi {
 
 // property_set implements a map of category-type to property-type.
 // for each category only one property in that category is allowed in the set.
-
-template <class T, class = void>
-struct property_traits;
-
-template <class T, class = void>
-struct property_set_traits;
-
-template <class... PropertyN>
-struct property_set;
-
-template <class T, class Target, class = void>
-struct property_set_traits_disable;
+//
 
 // customization point for a property with a category