Fix default arguments for orderBy()

Summary: Calling `folly::gen::orderBy()` without arguments causes a compilation error because the first template argument cannot be deduced. This diff fixes it.

Test Plan:
fbconfig -r folly && fbmake runtests

Reviewed By: philipp@fb.com

Subscribers: folly-diffs@, yfeldblum, chalfant

FB internal diff: D2025789

Signature: t1:2025789:1430167404:02fde7287b015d9dcbf398e8dc84cde7d74b4a5b

folly/gen/Base.h

@@ -625,24 +625,24 @@ Until until(Predicate pred = Predicate()) {
   return Until(std::move(pred));
 }
 
-template<class Selector,
+template<class Selector = Identity,
          class Comparer = Less,
          class Order = detail::Order<Selector, Comparer>>
-Order orderBy(Selector selector = Identity(),
+Order orderBy(Selector selector = Selector(),
               Comparer comparer = Comparer()) {
   return Order(std::move(selector),
                std::move(comparer));
 }
 
-template<class Selector,
+template<class Selector = Identity,
          class Order = detail::Order<Selector, Greater>>
-Order orderByDescending(Selector selector = Identity()) {
+Order orderByDescending(Selector selector = Selector()) {
   return Order(std::move(selector));
 }
 
-template<class Selector,
+template<class Selector = Identity,
          class Distinct = detail::Distinct<Selector>>
-Distinct distinctBy(Selector selector = Identity()) {
+Distinct distinctBy(Selector selector = Selector()) {
   return Distinct(std::move(selector));
 }
 

folly/gen/test/BaseTest.cpp

@@ -610,6 +610,14 @@ TEST(Gen, OrderBy) {
     | orderBy([](int x) { return (5.1 - x) * (5.1 - x); })
     | as<vector>();
   EXPECT_EQ(expected, actual);
+
+  expected = seq(1, 10) | as<vector>();
+  actual =
+      from(expected)
+    | map([] (int x) { return 11 - x; })
+    | orderBy()
+    | as<vector>();
+  EXPECT_EQ(expected, actual);
 }
 
 TEST(Gen, Foldl) {