Index: lib/Format/BreakableToken.cpp
===================================================================
--- lib/Format/BreakableToken.cpp
+++ lib/Format/BreakableToken.cpp
@@ -789,16 +789,47 @@
 
 void BreakableLineCommentSection::reflow(unsigned LineIndex,
                                          WhitespaceManager &Whitespaces) const {
-  // Reflow happens between tokens. Replace the whitespace between the
-  // tokens by the empty string.
-  Whitespaces.replaceWhitespace(
-      *Tokens[LineIndex], /*Newlines=*/0, /*Spaces=*/0,
-      /*StartOfTokenColumn=*/StartColumn, /*InPPDirective=*/false);
+  if (LineIndex > 0 && Tokens[LineIndex] != Tokens[LineIndex - 1]) {
+    // Reflow happens between tokens. Replace the whitespace between the
+    // tokens by the empty string.
+    Whitespaces.replaceWhitespace(
+        *Tokens[LineIndex], /*Newlines=*/0, /*Spaces=*/0,
+        /*StartOfTokenColumn=*/StartColumn, /*InPPDirective=*/false);
+  } else if (LineIndex > 0) {
+    // In case we're reflowing after the '\' in:
+    //
+    //   // line comment \
+    //   // line 2
+    //
+    // the reflow happens inside the single comment token (it is a single line
+    // comment with an unescaped newline).
+    // Replace the whitespace between the '\' and '//' with the empty string.
+    //
+    // Offset points to after the '\' relative to start of the token.
+    unsigned Offset = Lines[LineIndex - 1].data() +
+                      Lines[LineIndex - 1].size() -
+                      tokenAt(LineIndex - 1).TokenText.data();
+    // WhitespaceLength is the number of chars between the '\' and the '//' on
+    // the next line.
+    unsigned WhitespaceLength =
+        Lines[LineIndex].data() - tokenAt(LineIndex).TokenText.data() - Offset;
+    Whitespaces.replaceWhitespaceInToken(*Tokens[LineIndex],
+                                         Offset,
+                                         /*ReplaceChars=*/WhitespaceLength,
+                                         /*PreviousPostfix=*/"",
+                                         /*CurrentPrefix=*/"",
+                                         /*InPPDirective=*/false,
+                                         /*Newlines=*/0,
+                                         /*Spaces=*/0);
+
+  }
   // Replace the indent and prefix of the token with the reflow prefix.
+  unsigned Offset =
+      Lines[LineIndex].data() - tokenAt(LineIndex).TokenText.data();
   unsigned WhitespaceLength =
-      Content[LineIndex].data() - tokenAt(LineIndex).TokenText.data();
+      Content[LineIndex].data() - Lines[LineIndex].data();
   Whitespaces.replaceWhitespaceInToken(*Tokens[LineIndex],
-                                       /*Offset=*/0,
+                                       Offset,
                                        /*ReplaceChars=*/WhitespaceLength,
                                        /*PreviousPostfix=*/"",
                                        /*CurrentPrefix=*/ReflowPrefix,
Index: unittests/Format/FormatTestComments.cpp
===================================================================
--- unittests/Format/FormatTestComments.cpp
+++ unittests/Format/FormatTestComments.cpp
@@ -3090,6 +3090,21 @@
                getLLVMStyleWithColumns(23));
 }
 
+TEST_F(FormatTestComments, ReflowBackslashCrash) {
+// clang-format off
+  EXPECT_EQ(
+"// How to run:\n"
+"// bbbbb run \\\n"
+"// rrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr\n"
+"// \\ <log_file> -- --output_directory=\"<output_directory>\"",
+  format(
+"// How to run:\n"
+"// bbbbb run \\\n"
+"// rrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr \\\n"
+"// <log_file> -- --output_directory=\"<output_directory>\""));
+// clang-format on
+}
+
 } // end namespace
 } // end namespace format
 } // end namespace clang