diff --git a/clang-tools-extra/clangd/unittests/ParsedASTTests.cpp b/clang-tools-extra/clangd/unittests/ParsedASTTests.cpp
--- a/clang-tools-extra/clangd/unittests/ParsedASTTests.cpp
+++ b/clang-tools-extra/clangd/unittests/ParsedASTTests.cpp
@@ -667,7 +667,7 @@
   EXPECT_FALSE(mainIsGuarded(TU.build()));
 
   TU.Code = guard(";");
-  EXPECT_FALSE(mainIsGuarded(TU.build())); // FIXME: true
+  EXPECT_TRUE(mainIsGuarded(TU.build()));
 
   TU.Code = once(";");
   EXPECT_TRUE(mainIsGuarded(TU.build()));
@@ -768,7 +768,7 @@
   )cpp";
   AST = TU.build();
   EXPECT_THAT(*AST.getDiagnostics(), IsEmpty());
-  EXPECT_FALSE(mainIsGuarded(AST)); // FIXME: true
+  EXPECT_TRUE(mainIsGuarded(AST));
 
   // Guarded too late...
   TU.Code = R"cpp(
@@ -858,7 +858,7 @@
   TU.AdditionalFiles = {{"impl.h", Implementation}};
   auto AST = TU.build();
   EXPECT_THAT(*AST.getDiagnostics(), IsEmpty());
-  EXPECT_FALSE(mainIsGuarded(AST)); // FIXME: true
+  EXPECT_TRUE(mainIsGuarded(AST));
   // Slightly harder: the `#pragma once` is part of the preamble, and we
   // need to transfer it to the main file's HeaderFileInfo.
   TU.Code = once(Interface);
diff --git a/clang/lib/Lex/Lexer.cpp b/clang/lib/Lex/Lexer.cpp
--- a/clang/lib/Lex/Lexer.cpp
+++ b/clang/lib/Lex/Lexer.cpp
@@ -2793,6 +2793,11 @@
 
   if (PP->isRecordingPreamble() && PP->isInPrimaryFile()) {
     PP->setRecordedPreambleConditionalStack(ConditionalStack);
+    // If the preamble cuts off the end of a header guard, consider it guarded.
+    // The guard is valid for the preamble content itself, and for tools the
+    // most useful answer is "yes, this file has a header guard".
+    if (!ConditionalStack.empty())
+      MIOpt.ExitTopLevelConditional();
     ConditionalStack.clear();
   }