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[ttx] AttributeError "isCFF2" from T2CharString when dumping CFF2 toXML()

See original GitHub issue

Dumping “SourceSerifVariable-Roman.otf” with ttx yields an AttributeError from T2CharString.toXML method:

$ ttx ~/Downloads/SourceSerifVariable-Roman.otf 
Dumping "/home/clupo/Downloads/SourceSerifVariable-Roman.otf" to "/home/clupo/Downloads/SourceSerifVariable-Roman.ttx"...
Dumping 'GlyphOrder' table...
Dumping 'head' table...
Dumping 'hhea' table...
Dumping 'maxp' table...
Dumping 'OS/2' table...
Dumping 'hmtx' table...
Dumping 'cmap' table...
Dumping 'name' table...
Dumping 'post' table...
Dumping 'BASE' table...
Dumping 'CFF2' table...
ERROR: Unhandled exception has occurred
Traceback (most recent call last):
  File "/home/clupo/Github/fonttools/Lib/fontTools/ttx.py", line 398, in main
    process(jobs, options)
  File "/home/clupo/Github/fonttools/Lib/fontTools/ttx.py", line 372, in process
    action(input, output, options)
  File "/home/clupo/Github/fonttools/Lib/fontTools/misc/loggingTools.py", line 375, in wrapper
    return func(*args, **kwds)
  File "/home/clupo/Github/fonttools/Lib/fontTools/ttx.py", line 269, in ttDump
    newlinestr=options.newlinestr)
  File "/home/clupo/Github/fonttools/Lib/fontTools/ttLib/ttFont.py", line 228, in saveXML
    self._saveXML(writer, **kwargs)
  File "/home/clupo/Github/fonttools/Lib/fontTools/ttLib/ttFont.py", line 283, in _saveXML
    self._tableToXML(tableWriter, tag, splitGlyphs=splitGlyphs)
  File "/home/clupo/Github/fonttools/Lib/fontTools/ttLib/ttFont.py", line 314, in _tableToXML
    table.toXML(writer, self)
  File "/home/clupo/Github/fonttools/Lib/fontTools/ttLib/tables/C_F_F_.py", line 42, in toXML
    self.cff.toXML(writer)
  File "/home/clupo/Github/fonttools/Lib/fontTools/cffLib/__init__.py", line 156, in toXML
    font.toXML(xmlWriter)
  File "/home/clupo/Github/fonttools/Lib/fontTools/cffLib/__init__.py", line 2330, in toXML
    BaseDict.toXML(self, xmlWriter)
  File "/home/clupo/Github/fonttools/Lib/fontTools/cffLib/__init__.py", line 2272, in toXML
    conv.xmlWrite(xmlWriter, name, value)
  File "/home/clupo/Github/fonttools/Lib/fontTools/cffLib/__init__.py", line 1175, in xmlWrite
    value.toXML(xmlWriter)
  File "/home/clupo/Github/fonttools/Lib/fontTools/cffLib/__init__.py", line 722, in toXML
    self[i].toXML(xmlWriter)
  File "/home/clupo/Github/fonttools/Lib/fontTools/cffLib/__init__.py", line 2272, in toXML
    conv.xmlWrite(xmlWriter, name, value)
  File "/home/clupo/Github/fonttools/Lib/fontTools/cffLib/__init__.py", line 1175, in xmlWrite
    value.toXML(xmlWriter)
  File "/home/clupo/Github/fonttools/Lib/fontTools/cffLib/__init__.py", line 2272, in toXML
    conv.xmlWrite(xmlWriter, name, value)
  File "/home/clupo/Github/fonttools/Lib/fontTools/cffLib/__init__.py", line 1175, in xmlWrite
    value.toXML(xmlWriter)
  File "/home/clupo/Github/fonttools/Lib/fontTools/cffLib/__init__.py", line 652, in toXML
    subr.toXML(xmlWriter)
  File "/home/clupo/Github/fonttools/Lib/fontTools/misc/psCharStrings.py", line 1091, in toXML
    if self.isCFF2:
AttributeError: 'T2CharString' object has no attribute 'isCFF2'

If we need to parametrize the toXML method depending on whether isCFF2 or not, then we need to have an extra isCFF2=False parameter in the top-level CFFFontSet class, and set that to True from the table_C_F_F__2.toXML