From cf784212df2255844336b7ff0c7d36ee9037fa88 Mon Sep 17 00:00:00 2001
From: Tom Jackson <tjackson@fb.com>
Date: Thu, 8 Sep 2016 02:36:58 -0700
Subject: [PATCH] prvalues from get_ref_default()'s default functor

Summary: This previously allowed `get_ref_default(map, 4, []{ return 6; })`, even though this would form a reference to a temporary, then **use that invalid reference**.

Reviewed By: yfeldblum

Differential Revision: D3802707

fbshipit-source-id: 384d965f69c9d7b6bd3f011c8eff7fe55be7023a
---
 folly/MapUtil.h            | 4 +++-
 folly/test/MapUtilTest.cpp | 2 ++
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/folly/MapUtil.h b/folly/MapUtil.h
index 143dadc2..598567a7 100644
--- a/folly/MapUtil.h
+++ b/folly/MapUtil.h
@@ -116,7 +116,9 @@ template <
     typename Func,
     typename = typename std::enable_if<std::is_convertible<
         typename std::result_of<Func()>::type,
-        const typename Map::mapped_type&>::value>::type>
+        const typename Map::mapped_type&>::value>::type,
+    typename = typename std::enable_if<
+        std::is_reference<typename std::result_of<Func()>::type>::value>::type>
 const typename Map::mapped_type& get_ref_default(
     const Map& map,
     const typename Map::key_type& key,
diff --git a/folly/test/MapUtilTest.cpp b/folly/test/MapUtilTest.cpp
index ee1e7b5b..7ba12a98 100644
--- a/folly/test/MapUtilTest.cpp
+++ b/folly/test/MapUtilTest.cpp
@@ -85,6 +85,8 @@ TEST(MapUtil, get_ref_default_function) {
       std::addressof(i),
       std::addressof(
           get_ref_default(m, 2, [&i]() -> const int& { return i; })));
+  // statically disallowed:
+  // get_ref_default(m, 2, [] { return 7; });
 }
 
 TEST(MapUtil, get_ptr) {
-- 
2.34.1