diff --git a/src/google/protobuf/map.h b/src/google/protobuf/map.h
index 9dd425726d..37e19b0a7d 100644
--- a/src/google/protobuf/map.h
+++ b/src/google/protobuf/map.h
@@ -548,6 +548,10 @@ class Map {
     !defined(GOOGLE_PROTOBUF_OS_NACL) && !defined(GOOGLE_PROTOBUF_OS_ANDROID)
     template<class NodeType, class... Args>
     void construct(NodeType* p, Args&&... args) {
+      // Clang 3.6 doesn't compile static casting to void* directly. (Issue #1266)
+      // According C++ standard 5.2.9/1: "The static_cast operator shall not cast
+      // away constness". So first the maybe const pointer is casted to const void* and
+      // after the const void* is const casted.
       new (const_cast<void*>(static_cast<const void*>(p))) NodeType(std::forward<Args>(args)...);
     }