[issue20300] Argument Clinic raises exception for custom converter with default

[Ryan Smith-Roberts]

New submission from Ryan Smith-Roberts:

If a custom converter declares both default and converter class attributes, you get this exception (test file attached):

Exception raised during parsing:
Traceback (most recent call last):
  File "Tools/clinic/clinic.py", line 1445, in parse
    parser.parse(block)
  File "Tools/clinic/clinic.py", line 2738, in parse
    self.state(None)
  File "Tools/clinic/clinic.py", line 3468, in state_terminal
    self.function.docstring = self.format_docstring()
  File "Tools/clinic/clinic.py", line 3344, in format_docstring
    s += "".join(a)
TypeError: sequence item 2: expected str instance, NoneType found

----------
files: ac_converter_test.c
messages: 208448
nosy: Nikratio, larry, rmsr
priority: normal
severity: normal
status: open
title: Argument Clinic raises exception for custom converter with default
Added file: http://bugs.python.org/file33541/ac_converter_test.c

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue20300>
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