fix usage of disable_overload

include/boost/fiber/context.hpp

@@ -27,7 +27,6 @@
 
 #include <boost/fiber/detail/config.hpp>
 #include <boost/fiber/detail/decay_copy.hpp>
-#include <boost/fiber/detail/disable_overload.hpp>
 #include <boost/fiber/detail/fss.hpp>
 #include <boost/fiber/detail/spinlock.hpp>
 #include <boost/fiber/exceptions.hpp>
@@ -278,8 +277,7 @@ public:
     // worker fiber context
     template< typename StackAlloc,
               typename Fn,
-              typename Tpl,
-              typename = detail::disable_overload< context, Fn >
+              typename Tpl
     >
     context( worker_context_t,
              boost::context::preallocated palloc, StackAlloc salloc,

include/boost/fiber/fiber.hpp

@@ -56,8 +56,7 @@ public:
 
     template< typename StackAllocator,
               typename Fn,
-              typename ... Args,
-              typename = detail::disable_overload< fiber, Fn >
+              typename ... Args
     >
     fiber( std::allocator_arg_t, StackAllocator salloc, Fn && fn, Args && ... args) :
         impl_{ make_worker_context( salloc, std::forward< Fn >( fn), std::forward< Args >( args) ... ) } {

include/boost/fiber/future/packaged_task.hpp

@@ -48,8 +48,7 @@ public:
     }
 
     template< typename Fn,
-              typename Allocator,
-              typename = detail::disable_overload< packaged_task, Fn >
+              typename Allocator
     >
     explicit packaged_task( std::allocator_arg_t, Allocator const& alloc, Fn && fn) {
         typedef detail::task_object<