diff --git a/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp b/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp
--- a/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp
+++ b/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp
@@ -33,6 +33,7 @@
 using ::testing::Field;
 using ::testing::IsEmpty;
 using ::testing::Pair;
+using testing::SizeIs;
 using ::testing::UnorderedElementsAre;
 
 ::testing::Matcher<const Diag &> WithFix(::testing::Matcher<Fix> FixMatcher) {
@@ -378,6 +379,47 @@
       ElementsAre(Diag(Test.range(), "use of undeclared identifier 'b'")));
 }
 
+// Recursive main-file include is diagnosed, and doesn't crash.
+TEST(DiagnosticsTest, RecursivePreamble) {
+  auto TU = TestTU::withCode(R"cpp(
+    #include "foo.h" // error-ok
+    int symbol;
+  )cpp");
+  TU.Filename = "foo.h";
+  EXPECT_THAT(TU.build().getDiagnostics(),
+              ElementsAre(DiagName("pp_including_mainfile_in_preamble")));
+  EXPECT_THAT(TU.build().getLocalTopLevelDecls(), SizeIs(1));
+}
+
+// Recursive main-file include with #pragma once guard is OK.
+TEST(DiagnosticsTest, RecursivePreamblePragmaOnce) {
+  auto TU = TestTU::withCode(R"cpp(
+    #pragma once
+    #include "foo.h"
+    int symbol;
+  )cpp");
+  TU.Filename = "foo.h";
+  EXPECT_THAT(TU.build().getDiagnostics(), IsEmpty());
+  EXPECT_THAT(TU.build().getLocalTopLevelDecls(), SizeIs(1));
+}
+
+// Recursive main-file include with #ifndef guard should be OK.
+// However, it's not yet recognized (incomplete at end of preamble).
+TEST(DiagnosticsTest, RecursivePreambleIfndefGuard) {
+  auto TU = TestTU::withCode(R"cpp(
+    #ifndef FOO
+    #define FOO
+    #include "foo.h" // error-ok
+    int symbol;
+    #endif
+  )cpp");
+  TU.Filename = "foo.h";
+  // FIXME: should be no errors here.
+  EXPECT_THAT(TU.build().getDiagnostics(),
+              ElementsAre(DiagName("pp_including_mainfile_in_preamble")));
+  EXPECT_THAT(TU.build().getLocalTopLevelDecls(), SizeIs(1));
+}
+
 TEST(DiagnosticsTest, InsideMacros) {
   Annotations Test(R"cpp(
     #define TEN 10
diff --git a/clang/lib/Lex/PPDirectives.cpp b/clang/lib/Lex/PPDirectives.cpp
--- a/clang/lib/Lex/PPDirectives.cpp
+++ b/clang/lib/Lex/PPDirectives.cpp
@@ -1940,19 +1940,6 @@
     return {ImportAction::None};
   }
 
-  // Check for circular inclusion of the main file.
-  // We can't generate a consistent preamble with regard to the conditional
-  // stack if the main file is included again as due to the preamble bounds
-  // some directives (e.g. #endif of a header guard) will never be seen.
-  // Since this will lead to confusing errors, avoid the inclusion.
-  if (File && PreambleConditionalStack.isRecording() &&
-      SourceMgr.translateFile(&File->getFileEntry()) ==
-          SourceMgr.getMainFileID()) {
-    Diag(FilenameTok.getLocation(),
-         diag::err_pp_including_mainfile_in_preamble);
-    return {ImportAction::None};
-  }
-
   // Should we enter the source file? Set to Skip if either the source file is
   // known to have no effect beyond its effect on module visibility -- that is,
   // if it's got an include guard that is already defined, set to Import if it
@@ -2070,6 +2057,19 @@
     Action = (SuggestedModule && !getLangOpts().CompilingPCH) ? Import : Skip;
   }
 
+  // Check for circular inclusion of the main file.
+  // We can't generate a consistent preamble with regard to the conditional
+  // stack if the main file is included again as due to the preamble bounds
+  // some directives (e.g. #endif of a header guard) will never be seen.
+  // Since this will lead to confusing errors, avoid the inclusion.
+  if (Action == Enter && File && PreambleConditionalStack.isRecording() &&
+      SourceMgr.translateFile(&File->getFileEntry()) ==
+          SourceMgr.getMainFileID()) {
+    Diag(FilenameTok.getLocation(),
+         diag::err_pp_including_mainfile_in_preamble);
+    return {ImportAction::None};
+  }
+
   if (Callbacks && !IsImportDecl) {
     // Notify the callback object that we've seen an inclusion directive.
     // FIXME: Use a different callback for a pp-import?