gh-80448: argparse: Fix IndexError on store_true action (GH-15656)

(cherry picked from commit e02cc6d42a)

Co-authored-by: Hai Shi <shihai1992@gmail.com>
Co-authored-by: Rémi Lapeyre <remi.lapeyre@henki.fr>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>

Lib/argparse.py

@@ -1997,7 +1997,11 @@ def consume_optional(start_index):
                     # arguments, try to parse more single-dash options out
                     # of the tail of the option string
                     chars = self.prefix_chars
-                    if arg_count == 0 and option_string[1] not in chars:
+                    if (
+                        arg_count == 0
+                        and option_string[1] not in chars
+                        and explicit_arg != ''
+                    ):
                         action_tuples.append((action, [], option_string))
                         char = option_string[0]
                         option_string = char + explicit_arg[0]

Lib/test/test_argparse.py

@@ -296,7 +296,7 @@ class TestOptionalsSingleDashCombined(ParserTestCase):
         Sig('-z'),
     ]
     failures = ['a', '--foo', '-xa', '-x --foo', '-x -z', '-z -x',
-                '-yx', '-yz a', '-yyyx', '-yyyza', '-xyza']
+                '-yx', '-yz a', '-yyyx', '-yyyza', '-xyza', '-x=']
     successes = [
         ('', NS(x=False, yyy=None, z=None)),
         ('-x', NS(x=True, yyy=None, z=None)),

Misc/NEWS.d/next/Library/2019-09-03-15-45-19.bpo-36267.z42Ex7.rst

@@ -0,0 +1 @@
+Fix IndexError in :class:`argparse.ArgumentParser` when a ``store_true`` action is given an explicit argument.