Using pip fails with IndexError: index out of range: -1

[pekkaklarck]

After installing IronPython 2.7.7 and running ipy -X:Frames -m ensurepip, trying to install a package or just check the installed pip version fails like this:

C:\Windows\system32>ipy -X:Frames -m pip install robotframework
Unhandled exception:
Traceback (most recent call last):
  File "c:\Program Files (x86)\IronPython 2.7\Lib\runpy.py", line 182, in run_module
  File "c:\Program Files (x86)\IronPython 2.7\Lib\site-packages\pip\_vendor\pyparsing.py", line 1261, in setParseAction
  File "c:\Program Files (x86)\IronPython 2.7\Lib\site-packages\pip\_vendor\pyparsing.py", line 1043, in _trim_arity
  File "c:\Program Files (x86)\IronPython 2.7\Lib\runpy.py", line 111, in _get_module_details
  File "c:\Program Files (x86)\IronPython 2.7\Lib\site-packages\pip\__init__.py", line 26, in <module>
  File "c:\Program Files (x86)\IronPython 2.7\Lib\site-packages\pip\utils\__init__.py", line 27, in <module>
  File "c:\Program Files (x86)\IronPython 2.7\Lib\site-packages\pip\_vendor\pkg_resources\__init__.py", line 74, in <module>
  File "c:\Program Files (x86)\IronPython 2.7\Lib\site-packages\pip\_vendor\packaging\requirements.py", line 9, in <module>
  File "c:\Program Files (x86)\IronPython 2.7\Lib\site-packages\pip\_vendor\pyparsing.py", line 4715, in <module>
IndexError: index out of range: -1

Same problem if I just try to check installed pip version with --version or run the installed pip.exe directly.

[slozier]

I haven't been able to reproduce this using the steps provided (install IronPython 2.7.7, run ensure pip and then "try to do anything with pip").

After hitting the same issue while investigating #1573 I did boil it down to a simple repro case which does not invlove pyparsing: #1573 (comment)

[slide]

This issue was moved to IronLanguages/ironpython2#71