diff --git a/clang/include/clang/Basic/DiagnosticCommonKinds.td b/clang/include/clang/Basic/DiagnosticCommonKinds.td
--- a/clang/include/clang/Basic/DiagnosticCommonKinds.td
+++ b/clang/include/clang/Basic/DiagnosticCommonKinds.td
@@ -282,6 +282,10 @@
   "file '%0' modified since it was first processed">, DefaultFatal;
 def err_file_too_large : Error<
   "sorry, unsupported: file '%0' is too large for Clang to process">;
+def err_include_too_large : Error<
+  "sorry, this include generates a translation unit too large for"
+  " Clang to process. This may by a result from multiple"
+  " inclusions of unguarded header files.">, DefaultFatal;
 def err_unsupported_bom : Error<"%0 byte order mark detected in '%1', but "
   "encoding is not supported">, DefaultFatal;
 def err_unable_to_rename_temp : Error<
diff --git a/clang/lib/Basic/SourceManager.cpp b/clang/lib/Basic/SourceManager.cpp
--- a/clang/lib/Basic/SourceManager.cpp
+++ b/clang/lib/Basic/SourceManager.cpp
@@ -577,13 +577,18 @@
     SLocEntryLoaded[Index] = true;
     return FileID::get(LoadedID);
   }
+  unsigned FileSize = File->getSize();
+  if (!(NextLocalOffset + FileSize + 1 > NextLocalOffset &&
+        NextLocalOffset + FileSize + 1 <= CurrentLoadedOffset)) {
+    // From this point, there is no sensible way to point to the current
+    // source-location and say: This include at line ### generates a too
+    // big file, as the IncludePos received is
+    Diag.Report(IncludePos, diag::err_include_too_large);
+    return FileID();
+  }
   LocalSLocEntryTable.push_back(
       SLocEntry::get(NextLocalOffset,
                      FileInfo::get(IncludePos, File, FileCharacter, Filename)));
-  unsigned FileSize = File->getSize();
-  assert(NextLocalOffset + FileSize + 1 > NextLocalOffset &&
-         NextLocalOffset + FileSize + 1 <= CurrentLoadedOffset &&
-         "Ran out of source locations!");
   // We do a +1 here because we want a SourceLocation that means "the end of the
   // file", e.g. for the "no newline at the end of the file" diagnostic.
   NextLocalOffset += FileSize + 1;