Index: include/clang/Basic/DiagnosticLexKinds.td
===================================================================
--- include/clang/Basic/DiagnosticLexKinds.td
+++ include/clang/Basic/DiagnosticLexKinds.td
@@ -426,6 +426,8 @@
   "did not find header '%0' in framework '%1' (loaded from '%2')">;
 def err_pp_error_opening_file : Error<
   "error opening file '%0': %1">, DefaultFatal;
+def err_pp_including_mainfile_for_preamble : Error<
+  "main file cannot be included recursively when building a preamble">;
 def err_pp_empty_filename : Error<"empty filename">;
 def err_pp_include_too_deep : Error<"#include nested too deeply">;
 def err_pp_expects_filename : Error<"expected \"FILENAME\" or <FILENAME>">;
Index: lib/Basic/SourceManager.cpp
===================================================================
--- lib/Basic/SourceManager.cpp
+++ lib/Basic/SourceManager.cpp
@@ -1582,7 +1582,7 @@
     if (MainSLoc.isFile()) {
       const ContentCache *MainContentCache
         = MainSLoc.getFile().getContentCache();
-      if (!MainContentCache) {
+      if (!MainContentCache || !MainContentCache->OrigEntry) {
         // Can't do anything
       } else if (MainContentCache->OrigEntry == SourceFile) {
         FirstFID = MainFileID;
Index: lib/Lex/PPDirectives.cpp
===================================================================
--- lib/Lex/PPDirectives.cpp
+++ lib/Lex/PPDirectives.cpp
@@ -1871,6 +1871,18 @@
     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) == SourceMgr.getMainFileID()) {
+    Diag(FilenameTok.getLocation(),
+         diag::err_pp_including_mainfile_for_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
Index: test/Index/preamble-cyclic-include.cpp
===================================================================
--- /dev/null
+++ test/Index/preamble-cyclic-include.cpp
@@ -0,0 +1,9 @@
+// RUN: env CINDEXTEST_EDITING=1 c-index-test -test-annotate-tokens=%s:5:1:10:1 %s 2>&1 | FileCheck %s
+// CHECK-NOT: error: unterminated conditional directive
+// CHECK-NOT: Skipping: [4:1 - 8:7]
+// CHECK: error: main file cannot be included recursively when building a preamble
+#ifndef A_H
+#define A_H
+#  include "preamble-cyclic-include.cpp"
+int bar();
+#endif