clang-format: Fix merging of << at end of input.

[jpienaar]

Summary

Fix the way merging of LessLess is performed at the end of input. r229783 merged <,< in an incorrect manner at the end of input. This resulted in assert failures (found by fuzzer bot).

[djasper]

Looks good.

lib/Format/Format.cpp

666

Maybe call this FourthTokenIsLess.

lib/Format/Format.cpp

if (Style.Language == FormatStyle::LK_JavaScript) {
         return;
       if (tryMergeTokens(JSRightArrow))
         return;
     }
   }
 
   bool tryMergeLessLess() {
     // Merge X,less,less,Y into X,lessless,Y unless X or Y is less.
-    if (Tokens.size() < 4) {
-      // Merge <,<,eof to <<,eof
-      if (Tokens.back()->Tok.isNot(tok::eof))
-        return false;
+    if (Tokens.size() < 3)
 
-      auto &eof = Tokens.back();
-      Tokens.pop_back();
-      bool LessLessMerged;
-      if ((LessLessMerged = tryMergeTokens({tok::less, tok::less})))
-        Tokens.back()->Tok.setKind(tok::lessless);
-      Tokens.push_back(eof);
-      return LessLessMerged;
-    }
+    bool FourBackLess = false;
+    if (Tokens.size() > 3)
+      FourBackLess = (Tokens.end() - 4)[0]->is(tok::less);
 
-    auto First = Tokens.end() - 4;
-    if (First[3]->is(tok::less) || First[2]->isNot(tok::less) ||
-        First[1]->isNot(tok::less) || First[0]->is(tok::less))
+    auto First = Tokens.end() - 3;
+    if (First[2]->is(tok::less) || First[1]->isNot(tok::less) ||
+        First[0]->isNot(tok::less) || FourBackLess)
       return false;
 
     // Only merge if there currently is no whitespace between the two "<".
-    if (First[2]->WhitespaceRange.getBegin() !=
-        First[2]->WhitespaceRange.getEnd())
+    if (First[1]->WhitespaceRange.getBegin() !=
+        First[1]->WhitespaceRange.getEnd())
       return false;
 
-    First[1]->Tok.setKind(tok::lessless);
-    First[1]->TokenText = "<<";
-    First[1]->ColumnWidth += 1;
+    First[0]->Tok.setKind(tok::lessless);
+    First[0]->TokenText = "<<";
+    First[0]->ColumnWidth += 1;
     Tokens.erase(Tokens.end() - 2);
     return true;
   }
 
   bool tryMergeTokens(ArrayRef<tok::TokenKind> Kinds) {
     if (Tokens.size() < Kinds.size())
       return false;
 

unittests/Format/FormatTest.cpp

TEST_F(FormatTest, TripleAngleBrackets) {
   verifyFormat("f<1><<<1, 1>>>();");
   EXPECT_EQ("f<param><<<1, 1>>>();",
             format("f< param > <<< 1, 1 >>> ();"));
   verifyFormat("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
                "aaaaaaaaaaa<<<\n    1, 1>>>();");
 }
 
 TEST_F(FormatTest, MergeLessLessAtEnd) {
+  verifyFormat("<<");
   verifyFormat("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
                "aaallvm::outs() <<");
   verifyFormat("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
                "aaaallvm::outs()\n    <<");
 }
 
 TEST_F(FormatTest, HandleUnbalancedImplicitBracesAcrossPPBranches) {
   std::string code = "#if A\n"