diff --git a/llvm/include/llvm/ADT/STLArrayExtras.h b/llvm/include/llvm/ADT/STLArrayExtras.h
--- a/llvm/include/llvm/ADT/STLArrayExtras.h
+++ b/llvm/include/llvm/ADT/STLArrayExtras.h
@@ -26,6 +26,7 @@
 
 /// Find the length of an array.
 template <class T, std::size_t N>
+LLVM_DEPRECATED("Use std::size instead.", "std::size")
 constexpr inline size_t array_lengthof(T (&)[N]) {
   return N;
 }