Hello!

Scenario: I have packages atf-c, atf-c++, atf-sh. (Any set of two or more packages works for this trick; I happen to have these three from the same source alphabetically early in my list of packages.)

$ pkgconf --modversion atf-c atf-c++ atf-sh
0.20
0.20

... so where did the third one go?

Spelunking the code, I see that --modversion (and some other flags) put us into a mode where we only consider one package at a time by setting maximum_package_count — perfectly reasonable (though I don't know if this behaviour is actually documented anywhere except here in the source, so surprising in that regard):

However, the logic to actually enforce this erroneously compares cardinal pkgq.length to ordinal maximum_package_count:

(I stumbled across this bug because Cabal does this wrong then fails for mysterious reasons; I'll also file a bug there about this behaviour.)