Index: clang/lib/Format/UnwrappedLineParser.cpp
===================================================================
--- clang/lib/Format/UnwrappedLineParser.cpp
+++ clang/lib/Format/UnwrappedLineParser.cpp
@@ -837,6 +837,10 @@
   unsigned InitialLevel = Line->Level;
   nextToken(/*LevelDifference=*/AddLevels);
 
+  // Bail out if there are too many levels. Otherwise, the stack might overflow.
+  if (Line->Level > 300)
+    return IfStmtKind::NotIf;
+
   if (MacroBlock && FormatTok->is(tok::l_paren))
     parseParens();