Change requires to use a default template argument (nonstd-lite-proje…

…ct 40)

Fixes implementation of nsop_REQUIRES_T(), see martinmoene/nonstd-lite-project#40

include/nonstd/observer_ptr.hpp

@@ -252,18 +252,14 @@ namespace nonstd {
 // Method enabling
 
 #define nsop_REQUIRES_T(VA) \
-    , typename = typename std::enable_if< (VA), nonstd::observer_ptr_lite::detail::enabler >::type
+    , typename std::enable_if< (VA), int >::type = 0
 
 //
 // oberver_ptr:
 //
 
 namespace nonstd { namespace observer_ptr_lite {
 
-// for optional_REQUIRES_T
-
-namespace detail { /*enum*/ class enabler{}; }
-
 // observer_ptr:
 
 template< class W >