optional builtin not found when run from namespace

[McDutchie]

This bug depends on the value of PATH.

Reproducer (replace /path/to/ksh):

PATH=$(getconf PATH) /path/to/ksh -x -c \
    'builtin basename; namespace ns { whence -a basename; basename --version; }; basename --version'

Output (on my system):

+ builtin basename
+ whence -a basename
basename is a shell builtin
basename is /usr/bin/basename
+ basename --version
basename: illegal option -- -
usage: basename string [suffix]
       basename [-a] [-s suffix] string [...]
+ basename --version
  version         basename (ksh 93u+m) 2022-08-30

Expected output:

  version         basename (ksh 93u+m) 2022-08-30
  version         basename (ksh 93u+m) 2022-08-30

Within the namespace, the builtin was not found, and the default external basename was incorrectly run. This is in spite of whence -a correctly finding the built-in.

This bug is also in 93u+ 2012-08-01 and in ksh2020.