diff --git a/clang-tools-extra/clangd/index/Merge.cpp b/clang-tools-extra/clangd/index/Merge.cpp
--- a/clang-tools-extra/clangd/index/Merge.cpp
+++ b/clang-tools-extra/clangd/index/Merge.cpp
@@ -186,7 +186,10 @@
     S.Signature = O.Signature;
   if (S.CompletionSnippetSuffix == "")
     S.CompletionSnippetSuffix = O.CompletionSnippetSuffix;
-  if (S.Documentation == "")
+  // Don't accept documentation from bare forward declarations, if there is a
+  // definition and it didn't provide one. S is often an undocumented class,
+  // and O is a non-canonical forward decl preceded by an irrelevant comment.
+  if (S.Documentation == "" && !S.Definition)
     S.Documentation = O.Documentation;
   if (S.ReturnType == "")
     S.ReturnType = O.ReturnType;
diff --git a/clang-tools-extra/clangd/unittests/IndexTests.cpp b/clang-tools-extra/clangd/unittests/IndexTests.cpp
--- a/clang-tools-extra/clangd/unittests/IndexTests.cpp
+++ b/clang-tools-extra/clangd/unittests/IndexTests.cpp
@@ -413,6 +413,16 @@
                                        FileURI("unittest:///test2.cc"))))));
 }
 
+TEST(MergeIndexTest, NonDocumentation) {
+  Symbol L, R;
+  L.ID = R.ID = SymbolID("x");
+  L.Definition.FileURI = "file:/x.h";
+  R.Documentation = "Forward declarations because x.h is too big to include";
+
+  Symbol M = mergeSymbol(L, R);
+  EXPECT_EQ(M.Documentation, "");
+}
+
 MATCHER_P2(IncludeHeaderWithRef, IncludeHeader, References, "") {
   return (arg.IncludeHeader == IncludeHeader) && (arg.References == References);
 }