Fix assertion when breaking string literals with tab characters.

[alexfh]

Summary

Fixes http://llvm.org/PR19368

[klimek]

lg

lib/Format/ContinuationIndenter.cpp

while (RemainingTokenColumns > RemainingSpace) {
       // We can remove extra whitespace instead of breaking the line.
       if (RemainingTokenColumns + 1 - Split.second <= RemainingSpace) {
         RemainingTokenColumns = 0;
         if (!DryRun)
           Token->replaceWhitespace(LineIndex, TailOffset, Split, Whitespaces);
         break;
       }
 
+      // When breaking before a tab character, it may be moved by a few columns,
+      // but will still be expanded to the next tab stop, so we don't save any
+      // columns.
+      if (NewRemainingTokenColumns == RemainingTokenColumns)
+        break;
+
       assert(NewRemainingTokenColumns < RemainingTokenColumns);
       if (!DryRun)
         Token->insertBreak(LineIndex, TailOffset, Split, Whitespaces);
       Penalty += Current.SplitPenalty;
       unsigned ColumnsUsed =
           Token->getLineLengthAfterSplit(LineIndex, TailOffset, Split.first);
       if (ColumnsUsed > ColumnLimit) {
         Penalty += Style.PenaltyExcessCharacter * (ColumnsUsed - ColumnLimit);

unittests/Format/FormatTest.cpp

EXPECT_EQ(" void f() {\n"
                    "#define A \\\n"
                    "    int i;  \\\n"
                    "   int j;\n"
                    "      int k;\n" // Format this line.
                    " }",
                    67, 0, getLLVMStyle()));
 }
 
-TEST_F(FormatTest, BreakStringLiterals) {
+TEST_F(FormatTest, BreaksStringLiterals) {
   EXPECT_EQ("\"some text \"\n"
             "\"other\";",
             format("\"some text other\";", getLLVMStyleWithColumns(12)));
   EXPECT_EQ("\"some text \"\n"
             "\"other\";",
             format("\\\n\"some text other\";", getLLVMStyleWithColumns(12)));
   EXPECT_EQ(
       "#define A  \\\n"
TEST_F(FormatTest, BreaksStringLiterals) {
   EXPECT_EQ(
       "#define A \\\n"
       "  \"some \" \\\n"
       "  \"text \" \\\n"
       "  \"other\";",
       format("#define A \"some text other\";", AlignLeft));
 }
 
+TEST_F(FormatTest, BreaksStringLiteralsWithTabs) {
+  EXPECT_EQ(
+      "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
+      "(\n"
+      "    \"x\t\");",
+      format("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
+             "aaaaaaa("
+             "\"x\t\");"));
+}
+
 TEST_F(FormatTest, BreaksWideAndNSStringLiterals) {
   EXPECT_EQ(
       "u8\"utf8 string \"\n"
       "u8\"literal\";",
       format("u8\"utf8 string literal\";", getGoogleStyleWithColumns(16)));
   EXPECT_EQ(
       "u\"utf16 string \"\n"
       "u\"literal\";",