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

…ct 40)

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

include/nonstd/expected.hpp

@@ -241,7 +241,7 @@ namespace nonstd {
     template< bool B = (__VA_ARGS__), typename std::enable_if<B, int>::type = 0 >
 
 #define nsel_REQUIRES_T(...) \
-    , typename = typename std::enable_if< (__VA_ARGS__), nonstd::expected_lite::detail::enabler >::type
+    , typename std::enable_if< (__VA_ARGS__), int >::type = 0
 
 #define nsel_REQUIRES_R(R, ...) \
     typename std::enable_if< (__VA_ARGS__), R>::type
@@ -424,10 +424,6 @@ class expected;
 
 namespace detail {
 
-/// for nsel_REQUIRES_T
-
-enum class enabler{};
-
 /// discriminated union to hold value or 'error'.
 
 template< typename T, typename E >