[clang-format] Don't skip PP lines if original line was a PP line when trying to merge lines

[aeubanks]

Summary

Fixes a crash introduced in D123737 where LastNonComment would be null.

[aeubanks]

clang/unittests/Format/FormatTest.cpp

13397

not sure if you'd like a better test, this was the reduced test case I got that still caused a crash

[curdeius]

LGTM.

clang/unittests/Format/FormatTest.cpp

13397

That's ok for me.

[owenpan]

clang/lib/Format/UnwrappedLineFormatter.cpp

309–316

Or simply return before the for loop if TheLine->InPPDirective is true?

[aeubanks]

take suggestion

clang/lib/Format/UnwrappedLineFormatter.cpp

309–316

I'm not 100% sure how TheLine->Level works in the presence of macros (e.g. a function that's fully defined in a macro), would we want to also search for (*J)->Level < TheLine->Level in that case?

either way, I've taken your suggestion because I'd like to get this crash fix landed

clang/lib/Format/UnwrappedLineFormatter.cpp

auto ShouldMergeShortFunctions = [this, &I, &NextLine, PreviousLine,
           return true;
 
         if (TheLine->Level != 0) {
           if (!PreviousLine)
             return false;
 
           // TODO: Use IndentTracker to avoid loop?
           // Find the last line with lower level.
           auto J = I - 1;
-          for (; J != AnnotatedLines.begin(); --J)
+          if (!TheLine->InPPDirective) {
+            for (; J != AnnotatedLines.begin(); --J) {
               if (!(*J)->InPPDirective && (*J)->Level < TheLine->Level)
                 break;
+            }
+          }
+
           if ((*J)->Level >= TheLine->Level)
             return false;
 
           // Check if the found line starts a record.
           const FormatToken *LastNonComment = (*J)->Last;
           assert(LastNonComment);
           if (LastNonComment->is(tok::comment)) {
             LastNonComment = LastNonComment->getPreviousNonComment();

clang/unittests/Format/FormatTest.cpp

verifyFormat("#define A                \\\n"
                "    return 1;            \\\n"
                "  return 2;",
                ShortMergedIf);
   verifyFormat("#define A                \\\n"
                "  if CONSTEXPR (aaaaaaa) \\\n"
                "    return 1;            \\\n"
                "  return 2;",
                ShortMergedIf);
+
+  verifyFormat("//\n"
+               "#define a \\\n"
+               "  if      \\\n"
+               "  0",
+               getChromiumStyle(FormatStyle::LK_Cpp));
 }
 
 TEST_F(FormatTest, FormatStarDependingOnContext) {
   verifyFormat("void f(int *a);");
   verifyFormat("void f() { f(fint * b); }");
   verifyFormat("class A {\n  void f(int *a);\n};");
   verifyFormat("class A {\n  int *a;\n};");
   verifyFormat("namespace a {\n"