diff --git a/clang/lib/Format/UnwrappedLineParser.cpp b/clang/lib/Format/UnwrappedLineParser.cpp
--- a/clang/lib/Format/UnwrappedLineParser.cpp
+++ b/clang/lib/Format/UnwrappedLineParser.cpp
@@ -1215,9 +1215,12 @@
   // If a potential include guard has an #else, it's not an include guard.
   if (IncludeGuard == IG_Defined && PPBranchLevel == 0)
     IncludeGuard = IG_Rejected;
+  // Don't crash when there is an #else without an #if.
+  assert(PPBranchLevel >= -1);
+  if (PPBranchLevel == -1)
+    conditionalCompilationStart(/*Unreachable=*/true);
   conditionalCompilationAlternative();
-  if (PPBranchLevel > -1)
-    --PPBranchLevel;
+  --PPBranchLevel;
   parsePPUnknown();
   ++PPBranchLevel;
 }
diff --git a/clang/unittests/Format/FormatTest.cpp b/clang/unittests/Format/FormatTest.cpp
--- a/clang/unittests/Format/FormatTest.cpp
+++ b/clang/unittests/Format/FormatTest.cpp
@@ -5193,6 +5193,34 @@
   verifyNoCrash("a={0,1\n#if a\n#else\n;\n#endif\n}");
   verifyNoCrash("#if a\na(\n#else\n#endif\n) a {a,b,c,d,f,g};");
   verifyNoCrash("#ifdef A\n a(\n #else\n #endif\n) = []() {      \n)}");
+  verifyNoCrash("#else\n"
+                "#else\n"
+                "#endif\n"
+                "#endif");
+  verifyNoCrash("#else\n"
+                "#if X\n"
+                "#endif\n"
+                "#endif");
+  verifyNoCrash("#else\n"
+                "#endif\n"
+                "#if X\n"
+                "#endif");
+  verifyNoCrash("#if X\n"
+                "#else\n"
+                "#else\n"
+                "#endif\n"
+                "#endif");
+  verifyNoCrash("#if X\n"
+                "#elif Y\n"
+                "#elif Y\n"
+                "#endif\n"
+                "#endif");
+  verifyNoCrash("#endif\n"
+                "#endif");
+  verifyNoCrash("#endif\n"
+                "#else");
+  verifyNoCrash("#endif\n"
+                "#elif Y");
 }
 
 TEST_F(FormatTest, MacrosWithoutTrailingSemicolon) {