Fix forward_like rvalue->lvalue check

Summary: The static cast and forward were backwards. Previously it happily converted an rvalue to a const lvalue. Now it causes the exact same static_assert failure as std::forward in every rvalue->lvalue case.

Reviewed By: yfeldblum

Differential Revision: D26680263

fbshipit-source-id: f1ca4c4bfdd6333d24fe3c406d5373a2f72dea31

folly/Utility.h

@@ -106,7 +106,7 @@ void as_const(T const&&) = delete;
 //  mimic: forward_like, p0847r0
 template <typename Src, typename Dst>
 constexpr like_t<Src, Dst>&& forward_like(Dst&& dst) noexcept {
-  return static_cast<like_t<Src, Dst>&&>(std::forward<Dst>(dst));
+  return std::forward<like_t<Src, Dst>>(static_cast<Dst&&>(dst));
 }
 
 /**

folly/test/UtilityTest.cpp

@@ -81,6 +81,11 @@ TEST_F(UtilityTest, forward_like) {
   // the real work is done by like_t, in terms of which forward_like is defined
   EXPECT_EQ(&x, std::addressof(folly::forward_like<char&>(x)));
   EXPECT_EQ(&x, std::addressof(as_mutable(folly::forward_like<char const>(x))));
+
+  // Should not be able to turn rvalues into lvalues
+  // Uncomment to produce expected compile-time errors
+  // std::forward<const int&>(1);
+  // folly::forward_like<const int&>(1);
 }
 
 TEST_F(UtilityTest, MoveOnly) {