Make collect work for types with no default constructors

Summary: This doesn't make the code less efficient, because RVO can't be used for CollectVariadicContext. Thus moving existing tuple vs constructing new tuple by moving all values from other tuple (where each value is wrapped in folly::Optional) should be pretty much the same.

Reviewed By: hannesr

Differential Revision: D2650293

fb-gh-sync-id: 648a358bf093a0bb9d058a997af9bf59014ad77c

folly/futures/detail/Core.h

@@ -410,18 +410,37 @@ struct CollectVariadicContext {
          p.setException(std::move(t.exception()));
        }
      } else if (!threw) {
-       std::get<I>(results) = std::move(t.value());
+       std::get<I>(results) = std::move(t);
      }
   }
   ~CollectVariadicContext() {
     if (!threw.exchange(true)) {
-      p.setValue(std::move(results));
+      p.setValue(unwrap(std::move(results)));
     }
   }
   Promise<std::tuple<Ts...>> p;
-  std::tuple<Ts...> results;
+  std::tuple<folly::Try<Ts>...> results;
   std::atomic<bool> threw {false};
   typedef Future<std::tuple<Ts...>> type;
+
+ private:
+  template <typename... Ts2>
+  static std::tuple<Ts...> unwrap(std::tuple<folly::Try<Ts>...>&& o,
+                                  Ts2&&... ts2) {
+    static_assert(sizeof...(ts2) <
+                  std::tuple_size<std::tuple<folly::Try<Ts>...>>::value,
+                  "Non-templated unwrap should be used instead");
+    assert(std::get<sizeof...(ts2)>(o).hasValue());
+
+    return unwrap(std::move(o),
+                  std::forward<Ts2>(ts2)...,
+                  std::move(*std::get<sizeof...(ts2)>(o)));
+  }
+
+  static std::tuple<Ts...> unwrap(std::tuple<folly::Try<Ts>...>&& o,
+                                  Ts&&... ts) {
+    return std::tuple<Ts...>(std::forward<Ts>(ts)...);
+  }
 };
 
 template <template <typename ...> class T, typename... Ts>

folly/futures/test/CollectTest.cpp

@@ -630,3 +630,14 @@ TEST(Collect, collectAllNone) {
   auto f = collectAll(fs);
   EXPECT_TRUE(f.isReady());
 }
+
+TEST(Collect, noDefaultConstructor) {
+  struct A {
+    explicit A(size_t x) {}
+  };
+
+  auto f1 = makeFuture(A(1));
+  auto f2 = makeFuture(A(2));
+
+  auto f = collect(std::move(f1), std::move(f2));
+}