[llvm-bugs] [Bug 46964] New: Trailing comments after endif adds macro continuation

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=46964

            Bug ID: 46964
           Summary: Trailing comments after endif adds macro continuation
           Product: clang
           Version: 10.0
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: Formatter
          Assignee: unassignedclangbugs at nondot.org
          Reporter: geoffrey.carlton at activision.com
                CC: djasper at google.com, klimek at google.com,
                    llvm-bugs at lists.llvm.org

Created attachment 23809
  --> https://bugs.llvm.org/attachment.cgi?id=23809&action=edit
Input to be formatted with llvm and ColumnLimit 0

When an #endif or #else has a trailing comment, and a comment follows on the
next line, clang-format can add an inappropriate macro continuation '\\' at the
end of the first comment.

Minimum repro is attached.  Input file is attached, but is in full:
  "#if A()\n#endif // #if A()\n\t\t\t// hello\n"
Output becomes:
  "#if A()\n#endif // #if A() \\\n       // hello\n"

Reproduces in clang 10.0.0 with -style="{BasedOnStyle: llvm, ColumnLimit: 0}". 
Also reproduces in latest code built from github.

The code is related to ContinuationIndenter::addTokenOnNewLine where it is
adding whitespace with ContinuePPDirective set.  That causes the comment to be
joined with the macro continuation character.

A partial workaround is before that call to replaceWhitespace:

    if (State.Line->First->is(tok::hash) && Current.is(tok::comment) &&
        State.Line->First->Next != nullptr) {
      if (isIfdefType(State.Line->First->Next)) {
        // Don't count this trailing comment part of a PP directive or it will
        // unnecessarily escape the end with '\' If we happen to have a another
        // comment on the next line.
        ContinuePPDirective = false;
        // An additional problem is later parts of clang-format can shift
around
        // the following comment, causing the output not to be stable.
        Current.Finalized = true;
      }
    }
Where isIfdefType is checking for ifdef/elif/else/endif.  That's good enough
for a quick fix but isn't the real solution to the problem.

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