From 5a54977ea8ba690192d98c1fd4571858cc1451ac Mon Sep 17 00:00:00 2001
From: David Majnemer <david.majnemer@gmail.com>
Date: Sun, 14 Dec 2014 04:53:11 +0000
Subject: [PATCH] Lex: Don't crash if both conflict markers are on the same
 line

We would check if the terminator marker is on a newline.  However, the
logic would end up out-of-bounds if the terminator marker immediately
follows the start marker.

This fixes PR21820.

llvm-svn: 224210
---
 clang/lib/Lex/Lexer.cpp            | 4 ++--
 clang/test/Lexer/conflict-marker.c | 2 ++
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/clang/lib/Lex/Lexer.cpp b/clang/lib/Lex/Lexer.cpp
index c2e9716123c4..ca5252e1c9ce 100644
--- a/clang/lib/Lex/Lexer.cpp
+++ b/clang/lib/Lex/Lexer.cpp
@@ -2588,8 +2588,8 @@ static const char *FindConflictEnd(const char *CurPtr, const char *BufferEnd,
   size_t Pos = RestOfBuffer.find(Terminator);
   while (Pos != StringRef::npos) {
     // Must occur at start of line.
-    if (RestOfBuffer[Pos-1] != '\r' &&
-        RestOfBuffer[Pos-1] != '\n') {
+    if (Pos == 0 ||
+        (RestOfBuffer[Pos - 1] != '\r' && RestOfBuffer[Pos - 1] != '\n')) {
       RestOfBuffer = RestOfBuffer.substr(Pos+TermLen);
       Pos = RestOfBuffer.find(Terminator);
       continue;
diff --git a/clang/test/Lexer/conflict-marker.c b/clang/test/Lexer/conflict-marker.c
index e5bc7f33e45f..2611827d2ace 100644
--- a/clang/test/Lexer/conflict-marker.c
+++ b/clang/test/Lexer/conflict-marker.c
@@ -36,3 +36,5 @@ int foo() {
   y a = x;
   return x + a - z;
 }
+
+<<<<<<<>>>>>>> // expected-error {{expected identifier}}