Conditionally compiled for_each with constexpr

Summary:
Code was breaking in versions of C++ where constexpr functions are
required to have one line bodies

Reviewed By: yfeldblum, mzlee

Differential Revision: D5739062

fbshipit-source-id: 6c509f1daf77751d33ce9c173a0d7f1d3bd2a006

folly/Foreach-inl.h

@@ -22,6 +22,7 @@
 #include <type_traits>
 #include <utility>
 
+#include <folly/Portability.h>
 #include <folly/Traits.h>
 
 namespace folly {
@@ -378,14 +379,14 @@ struct FetchImpl<S, EnableIfRange<S>> {
 } // namespace for_each_detail
 
 template <typename Sequence, typename Func>
-constexpr Func for_each(Sequence&& range, Func func) {
+FOLLY_CPP14_CONSTEXPR Func for_each(Sequence&& range, Func func) {
   for_each_detail::ForEachImpl<typename std::decay<Sequence>::type>::impl(
       std::forward<Sequence>(range), func);
   return func;
 }
 
 template <typename Sequence, typename Index>
-constexpr decltype(auto) fetch(Sequence&& sequence, Index&& index) {
+FOLLY_CPP14_CONSTEXPR decltype(auto) fetch(Sequence&& sequence, Index&& index) {
   return for_each_detail::FetchImpl<Sequence>::impl(
       std::forward<Sequence>(sequence), std::forward<Index>(index));
 }

folly/Foreach.h

@@ -16,6 +16,7 @@
 
 #pragma once
 
+#include <folly/Portability.h>
 #include <folly/Preprocessor.h>
 
 #include <type_traits>
@@ -79,7 +80,7 @@ namespace folly {
  *  });
  */
 template <typename Range, typename Func>
-constexpr Func for_each(Range&& range, Func func);
+FOLLY_CPP14_CONSTEXPR Func for_each(Range&& range, Func func);
 
 /**
  * The user should return loop_break and loop_continue if they want to iterate
@@ -115,7 +116,7 @@ constexpr auto loop_continue = for_each_detail::LoopControl::CONTINUE;
  * (operator[]) to fetch the required element
  */
 template <typename Sequence, typename Index>
-constexpr decltype(auto) fetch(Sequence&& sequence, Index&& index);
+FOLLY_CPP14_CONSTEXPR decltype(auto) fetch(Sequence&& sequence, Index&& index);
 
 } // namespace folly