Index: clangd/CodeComplete.cpp
===================================================================
--- clangd/CodeComplete.cpp
+++ clangd/CodeComplete.cpp
@@ -44,6 +44,8 @@
 #include "clang/Format/Format.h"
 #include "clang/Frontend/CompilerInstance.h"
 #include "clang/Frontend/FrontendActions.h"
+#include "clang/Lex/ExternalPreprocessorSource.h"
+#include "clang/Lex/Preprocessor.h"
 #include "clang/Lex/PreprocessorOptions.h"
 #include "clang/Sema/CodeCompleteConsumer.h"
 #include "clang/Sema/DeclSpec.h"
@@ -1019,6 +1021,44 @@
   llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS;
 };
 
+void loadMainFilePreambleMacros(const Preprocessor &PP,
+                                llvm::StringRef PreambleText) {
+  // The ExternalPreprocessorSource has our macros, if we know where to look.
+  // We can read all the macros using PreambleMacros->ReadDefinedMacros(),
+  // but this includes transitively included files, so may deserialize a lot.
+  ExternalPreprocessorSource *PreambleMacros = PP.getExternalSource();
+  // If we have the names of the macros, we can look up their IdentifierInfo
+  // and then use this to load just the macros we want.
+  IdentifierInfoLookup *PreambleIdentifiers =
+      PP.getIdentifierTable().getExternalIdentifierLookup();
+  if (!PreambleIdentifiers || !PreambleMacros)
+    return;
+  auto Probe = [&](llvm::StringRef MacroName) {
+    if (auto *II = PreambleIdentifiers->get(MacroName))
+      PreambleMacros->updateOutOfDateIdentifier(*II);
+  };
+
+  // We get the identifiers by lexing the preamble text for #define lines.
+  // This may contain false positives (e.g. #if'ed out), it doesn't matter.
+  // This doesn't handle pathological input (e.g line continuation before name).
+  llvm::StringRef Line;
+  while (!PreambleText.empty()) {
+    std::tie(Line, PreambleText) = PreambleText.split('\n');
+    Line = Line.ltrim();
+    if (!Line.consume_front("#"))
+      continue;
+    Line = Line.ltrim();
+    if (!Line.consume_front("define"))
+      continue;
+    Line = Line.ltrim();
+    if (Line.empty() || !isIdentifierHead(Line.front()))
+      continue;
+    llvm::StringRef MacroName =
+        Line.take_while([&](char C) { return isIdentifierBody(C); });
+    Probe(MacroName);
+  }
+}
+
 // Invokes Sema code completion on a file.
 // If \p Includes is set, it will be updated based on the compiler invocation.
 bool semaCodeComplete(std::unique_ptr<CodeCompleteConsumer> Consumer,
@@ -1060,9 +1100,9 @@
   // However, if we're completing *inside* the preamble section of the draft,
   // overriding the preamble will break sema completion. Fortunately we can just
   // skip all includes in this case; these completions are really simple.
-  bool CompletingInPreamble =
-      ComputePreambleBounds(*CI->getLangOpts(), ContentsBuffer.get(), 0).Size >
-      Input.Offset;
+  PreambleBounds PreambleRegion =
+      ComputePreambleBounds(*CI->getLangOpts(), ContentsBuffer.get(), 0);
+  bool CompletingInPreamble = PreambleRegion.Size > Input.Offset;
   // NOTE: we must call BeginSourceFile after prepareCompilerInstance. Otherwise
   // the remapped buffers do not get freed.
   IgnoreDiagnostics DummyDiagsConsumer;
@@ -1080,6 +1120,14 @@
         Input.FileName);
     return false;
   }
+  // Macros can be defined within the preamble region of the main file.
+  // They don't fall nicely into our index/Sema dichotomy:
+  //  - they're not indexed for completion (they're not available across files)
+  //  - but Sema code complete won't see them: as part of the preamble, they're
+  //    deserialized only when mentioned.
+  // Force them to be deserialized so SemaCodeComplete sees them.
+  loadMainFilePreambleMacros(Clang->getPreprocessor(),
+                             Input.Contents.take_front(PreambleRegion.Size));
   if (Includes)
     Clang->getPreprocessor().addPPCallbacks(
         collectIncludeStructureCallback(Clang->getSourceManager(), Includes));
Index: clangd/index/SymbolCollector.cpp
===================================================================
--- clangd/index/SymbolCollector.cpp
+++ clangd/index/SymbolCollector.cpp
@@ -24,6 +24,7 @@
 #include "clang/Basic/SourceManager.h"
 #include "clang/Basic/Specifiers.h"
 #include "clang/Index/IndexSymbol.h"
+#include "clang/Index/IndexingAction.h"
 #include "clang/Index/USRGeneration.h"
 #include "clang/Lex/Preprocessor.h"
 #include "llvm/Support/Casting.h"
Index: unittests/clangd/CodeCompleteTests.cpp
===================================================================
--- unittests/clangd/CodeCompleteTests.cpp
+++ unittests/clangd/CodeCompleteTests.cpp
@@ -2069,19 +2069,28 @@
               UnorderedElementsAre(Named("Clangd_Macro_Test")));
 }
 
-TEST(CompletionTest, NoMacroFromPreambleIfIndexIsSet) {
+TEST(CompletionTest, MacroFromPreamble) {
+  MockFSProvider FS;
+  MockCompilationDatabase CDB;
+  std::string FooHeader = testPath("foo.h");
+  FS.Files[FooHeader] = "#define CLANGD_PREAMBLE_HEADER x\n";
+  IgnoreDiagnostics DiagConsumer;
+  ClangdServer Server(CDB, FS, DiagConsumer, ClangdServer::optsForTest());
   auto Results = completions(
-      R"cpp(#define CLANGD_PREAMBLE x
+      R"cpp(#include "foo.h"
+          define CLANGD_PREAMBLE_MAIN x
 
           int x = 0;
           #define CLANGD_MAIN x
           void f() { CLANGD_^ }
       )cpp",
       {func("CLANGD_INDEX")});
-  // Index is overriden in code completion options, so the preamble symbol is
-  // not seen.
-  EXPECT_THAT(Results.Completions, UnorderedElementsAre(Named("CLANGD_MAIN"),
-                                                        Named("CLANGD_INDEX")));
+  // We should get results from the main file, including the preamble section.
+  // However no results from included files (the index should cover them).
+  EXPECT_THAT(Results.Completions,
+              UnorderedElementsAre(Named("CLANGD_PREAMBLE_MAIN"),
+                                   Named("CLANGD_MAIN"),
+                                   Named("CLANGD_INDEX")));
 }
 
 TEST(CompletionTest, DeprecatedResults) {