Index: clang-tools-extra/clangd/index/dex/Iterator.cpp
===================================================================
--- clang-tools-extra/clangd/index/dex/Iterator.cpp
+++ clang-tools-extra/clangd/index/dex/Iterator.cpp
@@ -38,6 +38,8 @@
   /// or higher than the given one.
   void advanceTo(DocID ID) override {
     assert(!reachedEnd() && "DOCUMENT iterator can't advance() at the end.");
+    if (peek() >= ID)
+      return;
     Index = std::lower_bound(Index, std::end(Documents), ID);
   }