bug#72524: how does grep determine locale if no LC environment variables are set

[mark.yagnatinsky]

I ran into an odd issue... the workaround is easy enough but the issue is weird.
In case this relevant, my grep coms from git bash.  (which I think is mostly 
Cygwin? (or maybe msys2??))
Anyway, grep -P doesn't work if no LC vars are set, and complains that it only 
works in unibyte locales or UTF-8.
Normally, the git bash mintty launcher sets LC_CTYPE to en_us.UTF-8 but not if 
I bypass the launcher and run grep directly.
Here's the weird part, if I ask /usr/bin/locale what LC_TYPE "should" be, it 
says C.UTF-8.
If I run grep with C.UTF-8 then it also works.  So it must be deriving a 
default locale an different way.
But how?

Thanks for any pointers.
Mark.

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