Index: llvm/include/llvm/ADT/iterator.h
===================================================================
--- llvm/include/llvm/ADT/iterator.h
+++ llvm/include/llvm/ADT/iterator.h
@@ -114,6 +114,13 @@
     return !static_cast<const DerivedT *>(this)->operator==(RHS);
   }
 
+  bool operator<(const DerivedT &RHS) const {
+    static_assert(
+        IsRandomAccess,
+        "Relational operators are only defined for random access iterators.");
+    return !static_cast<const DerivedT *>(this)->operator>(RHS) &&
+           !static_cast<const DerivedT *>(this)->operator==(RHS);
+  }
   bool operator>(const DerivedT &RHS) const {
     static_assert(
         IsRandomAccess,