handle MSVC debug iterator tracking in F14 fallback

Summary:
MSVC's maintains an intrusive linked list of iterators for
debugging purposes in some build configurations, which means that
F14MapFallback's hacky conversion from unordered_map::local_iterator to
unordered_map::iterator doesn't work.  Fortunately, on that platform
local_iterator and iterator are the same type, so no hack is needed.

Reviewed By: Orvid

Differential Revision: D22776318

fbshipit-source-id: 80283c4c451afffe522e55e47cd94971126a6fa6

folly/container/detail/F14MapFallback.h

@@ -211,7 +211,7 @@ class F14BasicMap : public std::unordered_map<K, M, H, E, A> {
               return std::make_pair(it, false);
             }
             auto rv = Super::emplace(std::forward<decltype(inner)>(inner)...);
-            FOLLY_SAFE_CHECK(
+            FOLLY_SAFE_DCHECK(
                 rv.second, "post-find emplace should always insert");
             return rv;
           } else {
@@ -289,7 +289,6 @@ class F14BasicMap : public std::unordered_map<K, M, H, E, A> {
   template <typename K2>
   struct BottomKeyEqual {
     [[noreturn]] bool operator()(K2 const&, K2 const&) const {
-      FOLLY_SAFE_CHECK(false, "bucket should not invoke key equality");
       assume_unreachable();
     }
   };
@@ -379,19 +378,33 @@ class F14BasicMap : public std::unordered_map<K, M, H, E, A> {
   }
 
  private:
+  template <typename Iter, typename LocalIter>
+  static std::
+      enable_if_t<std::is_constructible<Iter, LocalIter const&>::value, Iter>
+      fromLocal(LocalIter const& src, int = 0) {
+    return Iter(src);
+  }
+
+  template <typename Iter, typename LocalIter>
+  static std::
+      enable_if_t<!std::is_constructible<Iter, LocalIter const&>::value, Iter>
+      fromLocal(LocalIter const& src) {
+    Iter dst;
+    static_assert(sizeof(dst) <= sizeof(src), "");
+    std::memcpy(std::addressof(dst), std::addressof(src), sizeof(dst));
+    FOLLY_SAFE_CHECK(
+        std::addressof(*src) == std::addressof(*dst),
+        "ABI-assuming local_iterator to iterator conversion failed");
+    return dst;
+  }
+
   template <typename Iter, typename Self, typename K2>
   static Iter findImpl(Self& self, K2 const& key) {
     auto optLocalIt = findLocal(self, key);
     if (!optLocalIt) {
       return self.end();
     } else {
-      Iter it;
-      static_assert(sizeof(it) <= sizeof(*optLocalIt), "");
-      std::memcpy(&it, &*optLocalIt, sizeof(it));
-      FOLLY_SAFE_CHECK(
-          std::addressof(**optLocalIt) == std::addressof(*it),
-          "ABI-assuming local_iterator to iterator conversion failed");
-      return it;
+      return fromLocal<Iter>(*optLocalIt);
     }
   }
 

folly/container/detail/F14SetFallback.h

@@ -116,7 +116,7 @@ class F14BasicSet
               return std::make_pair(it, false);
             }
             auto rv = Super::emplace(std::forward<decltype(key)>(key));
-            FOLLY_SAFE_CHECK(
+            FOLLY_SAFE_DCHECK(
                 rv.second, "post-find emplace should always insert");
             return rv;
           } else {
@@ -150,11 +150,30 @@ class F14BasicSet
   template <typename K>
   struct BottomKeyEqual {
     [[noreturn]] bool operator()(K const&, K const&) const {
-      FOLLY_SAFE_CHECK(false, "bucket should not invoke key equality");
       assume_unreachable();
     }
   };
 
+  template <typename Iter, typename LocalIter>
+  static std::
+      enable_if_t<std::is_constructible<Iter, LocalIter const&>::value, Iter>
+      fromLocal(LocalIter const& src, int = 0) {
+    return Iter(src);
+  }
+
+  template <typename Iter, typename LocalIter>
+  static std::
+      enable_if_t<!std::is_constructible<Iter, LocalIter const&>::value, Iter>
+      fromLocal(LocalIter const& src) {
+    Iter dst;
+    static_assert(sizeof(dst) <= sizeof(src), "");
+    std::memcpy(std::addressof(dst), std::addressof(src), sizeof(dst));
+    FOLLY_SAFE_CHECK(
+        std::addressof(*src) == std::addressof(*dst),
+        "ABI-assuming local_iterator to iterator conversion failed");
+    return dst;
+  }
+
   template <typename Iter, typename Self, typename K>
   static Iter findImpl(Self& self, K const& key) {
     if (self.empty()) {
@@ -171,13 +190,7 @@ class F14BasicSet
     auto e = self.end(slot);
     while (b != e) {
       if (self.key_eq()(key, *b)) {
-        Iter it;
-        static_assert(sizeof(it) <= sizeof(b), "");
-        std::memcpy(&it, &b, sizeof(it));
-        FOLLY_SAFE_CHECK(
-            std::addressof(*b) == std::addressof(*it),
-            "ABI-assuming local_iterator to iterator conversion failed");
-        return it;
+        return fromLocal<Iter>(b);
       }
       ++b;
     }