Make future_then_result instantiation lazy

STEllAR-GROUP · Mar 17, 2020 · 0aefb09 · 0aefb09
1 parent ab245a4
commit 0aefb09
Showing 1 changed file with 19 additions and 4 deletions.
diff --git a/hpx/lcos/future.hpp b/hpx/lcos/future.hpp
@@ -950,7 +950,10 @@ namespace hpx { namespace lcos
         using base_type::has_exception;
 
         template <typename F>
-        typename hpx::traits::future_then_result<future, F>::type
+        typename util::lazy_enable_if<
+            hpx::traits::is_invocable<F, future&&>::value,
+            hpx::traits::future_then_result<future, F>
+        >::type
         then(F && f, error_code& ec = throws)
         {
             invalidate on_exit(*this);
@@ -959,7 +962,12 @@ namespace hpx { namespace lcos
         }
 
         template <typename T0, typename F>
-        decltype(auto) then(T0 && t0, F && f, error_code& ec = throws)
+        typename util::lazy_enable_if<
+            !hpx::traits::is_invocable<T0, future&&>::value &&
+            hpx::traits::is_invocable<F, future&&>::value,
+            hpx::traits::future_then_result<future, F>
+        >::type
+        then(T0 && t0, F && f, error_code& ec = throws)
         {
             invalidate on_exit(*this);
             return base_type::then(std::move(*this),
@@ -1242,15 +1250,22 @@ namespace hpx { namespace lcos
         using base_type::has_exception;
 
         template <typename F>
-        typename hpx::traits::future_then_result<shared_future, F>::type
+        typename util::lazy_enable_if<
+            hpx::traits::is_invocable<F, shared_future&&>::value,
+            hpx::traits::future_then_result<shared_future, F>
+        >::type
         then(F && f, error_code& ec = throws) const
         {
             return base_type::then(shared_future(*this),
                 std::forward<F>(f), ec);
         }
 
         template <typename T0, typename F>
-        decltype(auto) then(T0 && t0, F && f, error_code& ec = throws) const
+        typename util::lazy_enable_if<
+            !hpx::traits::is_invocable<T0, shared_future&&>::value,
+            hpx::traits::future_then_result<shared_future, F>
+        >::type
+        then(T0 && t0, F && f, error_code& ec = throws) const
         {
             return base_type::then(shared_future(*this),
                 std::forward<T0>(t0), std::forward<F>(f), ec);