Fix strict aliasing violation in node_hash_map

node_hash_map uses the function TypeErasedDerefAndApplyToSlotFn to hash a
slot's key. This function accesses a slot pointer as if it were of type
Key* when in fact it is of type value_type* (aka std::pair<const Key,
Value> *). This is technically a strict aliasing violation because the
types of the pointer objects are different.  Fix it by defining a new
type erased function TypeErasedDerefAndApplyToSlotFirstFn that uses the
correct type for the pointer.

PiperOrigin-RevId: 918067685
Change-Id: I5d65e0e077f84a0f869be0e24fee094b4b09fbfb

absl/container/internal/container_memory.h

@@ -550,10 +550,20 @@ template <class Fn, class T, bool kIsDefault>
 size_t TypeErasedDerefAndApplyToSlotFn(const void* fn, void* slot_ptr,
                                        size_t seed) {
   const auto* f = static_cast<const Fn*>(fn);
-  const T* slot = *static_cast<const T**>(slot_ptr);
+  const T* slot = *static_cast<T**>(slot_ptr);
   return HashElement<Fn, kIsDefault>{*f, seed}(*slot);
 }
 
+// Type erased function to apply `Fn` to data inside of the `slot_ptr->first`.
+// The data is expected to have type `T`.
+template <class Fn, class T, bool kIsDefault>
+size_t TypeErasedDerefAndApplyToSlotFirstFn(const void* fn, void* slot_ptr,
+                                            size_t seed) {
+  const auto* f = static_cast<const Fn*>(fn);
+  const T* slot = *static_cast<T**>(slot_ptr);
+  return HashElement<Fn, kIsDefault>{*f, seed}(slot->first);
+}
+
 }  // namespace container_internal
 ABSL_NAMESPACE_END
 }  // namespace absl

absl/container/node_hash_map.h

@@ -681,7 +681,8 @@ class NodeHashMapPolicy
   template <class Hash, bool kIsDefault>
   static constexpr HashSlotFn get_hash_slot_fn() {
     return memory_internal::IsLayoutCompatible<Key, Value>::value
-               ? &TypeErasedDerefAndApplyToSlotFn<Hash, Key, kIsDefault>
+               ? &TypeErasedDerefAndApplyToSlotFirstFn<Hash, value_type,
+                                                       kIsDefault>
                : nullptr;
   }
 };