Index: clang-tools-extra/clangd/test/modules-options-compatablity-test/A.h
===================================================================
--- /dev/null
+++ clang-tools-extra/clangd/test/modules-options-compatablity-test/A.h
@@ -0,0 +1,2 @@
+// Comment for void a()
+void a() {}
Index: clang-tools-extra/clangd/test/modules-options-compatablity-test/B.h
===================================================================
--- /dev/null
+++ clang-tools-extra/clangd/test/modules-options-compatablity-test/B.h
@@ -0,0 +1,3 @@
+// Comment for B
+// ORDINARY COMMENT
+#include "A.h"
Index: clang-tools-extra/clangd/test/modules-options-compatablity-test/compile_commands.json
===================================================================
--- /dev/null
+++ clang-tools-extra/clangd/test/modules-options-compatablity-test/compile_commands.json
@@ -0,0 +1,8 @@
+[{
+  "directory" : ".",
+  "command" : "clang -emit-obj DIRECTORY/use.c -fno-modules-global -fmodules "
+              "-fmodules-cache-path=/DOES/NOT/EXIST  -fno-builtin-module-map "
+              "-fno-implicit-modules  -fimplicit-module-maps "
+              "-fprebuilt-module-path=DIRECTORY/prebuilt",
+  "file" : "DIRECTORY/use.c"
+}]
Index: clang-tools-extra/clangd/test/modules-options-compatablity-test/definition.jsonrpc
===================================================================
--- /dev/null
+++ clang-tools-extra/clangd/test/modules-options-compatablity-test/definition.jsonrpc
@@ -0,0 +1,28 @@
+{
+  "jsonrpc": "2.0",
+    "id": 0,
+    "method": "initialize",
+    "params": {
+      "processId": 123,
+      "rootPath": "clangd",
+      "capabilities": { "window": { "workDoneProgress": true, "implicitWorkDoneProgressCreate": true} },
+      "trace": "off"
+    }
+}
+---
+{
+  "jsonrpc": "2.0",
+    "method": "textDocument/didOpen",
+    "params": {
+      "textDocument": {
+        "uri": "file://DIRECTORY/use.c",
+        "languageId": "cpp",
+        "version": 1,
+        "text": "#include \"B.h\"\nvoid use(){\na();\n}"
+      }
+    }
+}
+---
+{"jsonrpc":"2.0","id":3,"method":"shutdown"}
+---
+{"jsonrpc":"2.0","method":"exit"}
Index: clang-tools-extra/clangd/test/modules-options-compatablity-test/module.modulemap
===================================================================
--- /dev/null
+++ clang-tools-extra/clangd/test/modules-options-compatablity-test/module.modulemap
@@ -0,0 +1,8 @@
+/* module.modulemap */
+module A {
+  header "A.h"
+}
+module B {
+  header "B.h"
+  export *
+}
Index: clang-tools-extra/clangd/test/modules-options-compatablity-test/use.c
===================================================================
--- /dev/null
+++ clang-tools-extra/clangd/test/modules-options-compatablity-test/use.c
@@ -0,0 +1,5 @@
+/* use */
+#include <A.h>
+#include <B.h>
+
+void use() { a(); }
Index: clang-tools-extra/clangd/test/modules-options-compatablity.test
===================================================================
--- /dev/null
+++ clang-tools-extra/clangd/test/modules-options-compatablity.test
@@ -0,0 +1,10 @@
+
+# RUN: rm -rf %t && mkdir -p %t
+# RUN: cp -r %S/modules-options-compatablity-test/* %t
+# RUN: mkdir -p %t/prebuilt
+# RUN: sed -i -e "s|DIRECTORY|%t|g" %t/definition.jsonrpc
+# RUN: sed -i -e "s|DIRECTORY|%t|g" %t/compile_commands.json
+# RUN: %clang -cc1 -emit-module -o %t/prebuilt/A.pcm -fmodules %t/module.modulemap -fmodule-name=A
+# RUN: %clang -cc1 -fretain-comments-from-system-headers -emit-module -o %t/prebuilt/B.pcm -fmodules %t/module.modulemap -fmodule-name=B -fprebuilt-module-path=%t/prebuilt
+# RUN: rm %t/*.h
+# RUN: clangd -background-index -lit-test < %t/definition.jsonrpc
Index: clang/include/clang/Basic/LangOptions.def
===================================================================
--- clang/include/clang/Basic/LangOptions.def
+++ clang/include/clang/Basic/LangOptions.def
@@ -387,7 +387,7 @@
 
 LANGOPT(XLPragmaPack, 1, 0, "IBM XL #pragma pack handling")
 
-LANGOPT(RetainCommentsFromSystemHeaders, 1, 0, "retain documentation comments from system headers in the AST")
+COMPATIBLE_LANGOPT(RetainCommentsFromSystemHeaders, 1, 0, "retain documentation comments from system headers in the AST")
 
 LANGOPT(SanitizeAddressFieldPadding, 2, 0, "controls how aggressive is ASan "
                                            "field padding (0: none, 1:least "
Index: clang/lib/Serialization/ASTReader.cpp
===================================================================
--- clang/lib/Serialization/ASTReader.cpp
+++ clang/lib/Serialization/ASTReader.cpp
@@ -3077,6 +3077,10 @@
           return Err;
         if (llvm::Error Err = ReadBlockAbbrevs(C, COMMENTS_BLOCK_ID))
           return Err;
+        if (PP.getPreprocessorOpts().GeneratePreamble &&
+            !PP.getPreprocessorOpts().WriteCommentListToPCH) {
+          break;
+        }
         CommentsCursors.push_back(std::make_pair(C, &F));
         break;
       }