From d5be1550f1406844501ab87a3e92622927a203e0 Mon Sep 17 00:00:00 2001
From: sstwcw <f0gukp2nk@protonmail.com>
Date: Sun, 30 Oct 2022 02:18:58 +0000
Subject: [PATCH] [clang-format] Don't crash on malformed preprocessor
 conditions

Previously the program would crash on this input:

```
#else
#if X
```

The problem was that in `parsePPElse`, `PPBranchLevel` would be
incremented when the first `#else` gets parsed, but `PPLevelBranchCount`
and `PPLevelBranchIndex` would not be updated together.

I found the problem when working on D135740.

Differential Revision: https://reviews.llvm.org/D135972
---
 clang/lib/Format/UnwrappedLineParser.cpp |  7 +++++--
 clang/unittests/Format/FormatTest.cpp    | 28 ++++++++++++++++++++++++++++
 2 files changed, 33 insertions(+), 2 deletions(-)

diff --git a/clang/lib/Format/UnwrappedLineParser.cpp b/clang/lib/Format/UnwrappedLineParser.cpp
index 25d9018..0372b89 100644
--- a/clang/lib/Format/UnwrappedLineParser.cpp
+++ b/clang/lib/Format/UnwrappedLineParser.cpp
@@ -1215,9 +1215,12 @@ void UnwrappedLineParser::parsePPElse() {
   // 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
index 69bd395..e2eced4 100644
--- a/clang/unittests/Format/FormatTest.cpp
+++ b/clang/unittests/Format/FormatTest.cpp
@@ -5193,6 +5193,34 @@ TEST_F(FormatTest, MacroDefinitionsWithIncompleteCode) {
   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) {
-- 
2.7.4