Resolve the circular dependency between folly/futures/detail/Core.h and Future.h and Promise.h

Summary: Do this by moving the two helpers that need to refer to them into `folly/futures/helpers.h`.

Reviewed By: yfeldblum

Differential Revision: D5179109

fbshipit-source-id: bdb319ff3432d3629a955c1390dc8bf6f27f4ce6

folly/futures/detail/Core.h

@@ -27,8 +27,7 @@
 #include <folly/Optional.h>
 #include <folly/ScopeGuard.h>
 #include <folly/Try.h>
-#include <folly/futures/Future.h>
-#include <folly/futures/Promise.h>
+#include <folly/futures/FutureException.h>
 #include <folly/futures/detail/FSM.h>
 
 #include <folly/io/async/Request.h>
@@ -422,45 +421,6 @@ class Core final {
   std::function<void(exception_wrapper const&)> interruptHandler_ {nullptr};
 };
 
-template <typename... Ts>
-struct CollectAllVariadicContext {
-  CollectAllVariadicContext() {}
-  template <typename T, size_t I>
-  inline void setPartialResult(Try<T>& t) {
-    std::get<I>(results) = std::move(t);
-  }
-  ~CollectAllVariadicContext() {
-    p.setValue(std::move(results));
-  }
-  Promise<std::tuple<Try<Ts>...>> p;
-  std::tuple<Try<Ts>...> results;
-  typedef Future<std::tuple<Try<Ts>...>> type;
-};
-
-template <typename... Ts>
-struct CollectVariadicContext {
-  CollectVariadicContext() {}
-  template <typename T, size_t I>
-  inline void setPartialResult(Try<T>& t) {
-    if (t.hasException()) {
-       if (!threw.exchange(true)) {
-         p.setException(std::move(t.exception()));
-       }
-     } else if (!threw) {
-       std::get<I>(results) = std::move(t);
-     }
-  }
-  ~CollectVariadicContext() noexcept {
-    if (!threw.exchange(true)) {
-      p.setValue(unwrapTryTuple(std::move(results)));
-    }
-  }
-  Promise<std::tuple<Ts...>> p;
-  std::tuple<folly::Try<Ts>...> results;
-  std::atomic<bool> threw {false};
-  typedef Future<std::tuple<Ts...>> type;
-};
-
 template <template <typename...> class T, typename... Ts>
 void collectVariadicHelper(const std::shared_ptr<T<Ts...>>& /* ctx */) {
   // base case

folly/futures/helpers.h

@@ -15,11 +15,58 @@
  */
 #pragma once
 
-#include <folly/futures/Future.h>
+#include <atomic>
+#include <tuple>
+#include <utility>
+
 #include <folly/Portability.h>
+#include <folly/Try.h>
+#include <folly/futures/Future.h>
+#include <folly/futures/Promise.h>
 
 namespace folly {
 
+namespace detail {
+template <typename... Ts>
+struct CollectAllVariadicContext {
+  CollectAllVariadicContext() {}
+  template <typename T, size_t I>
+  inline void setPartialResult(Try<T>& t) {
+    std::get<I>(results) = std::move(t);
+  }
+  ~CollectAllVariadicContext() {
+    p.setValue(std::move(results));
+  }
+  Promise<std::tuple<Try<Ts>...>> p;
+  std::tuple<Try<Ts>...> results;
+  typedef Future<std::tuple<Try<Ts>...>> type;
+};
+
+template <typename... Ts>
+struct CollectVariadicContext {
+  CollectVariadicContext() {}
+  template <typename T, size_t I>
+  inline void setPartialResult(Try<T>& t) {
+    if (t.hasException()) {
+      if (!threw.exchange(true)) {
+        p.setException(std::move(t.exception()));
+      }
+    } else if (!threw) {
+      std::get<I>(results) = std::move(t);
+    }
+  }
+  ~CollectVariadicContext() noexcept {
+    if (!threw.exchange(true)) {
+      p.setValue(unwrapTryTuple(std::move(results)));
+    }
+  }
+  Promise<std::tuple<Ts...>> p;
+  std::tuple<folly::Try<Ts>...> results;
+  std::atomic<bool> threw{false};
+  typedef Future<std::tuple<Ts...>> type;
+};
+}
+
 /// This namespace is for utility functions that would usually be static
 /// members of Future, except they don't make sense there because they don't
 /// depend on the template type (rather, on the type of their arguments in