inconsistent output file behaviour in qmake

The behaviour of qmake's -o switch differs between Windows vcxproj output and Mac xcodeproj output.

On Windows you must specify the extension:
qmake -tp vc -o foo bar.pro => generates foo (wrong)
qmake -tp vc -o foo.vcxproj bar.pro => generates foo.vcxproj (correct)

On Mac, you must omit the extension:
qmake -spec macx-xcode -o foo bar.pro => foo.xcodeproj (correct)
qmake -spec macx-xcode -o foo.xcodeproj bar.pro => foo.xcodeproj (correct, except... run it again

qmake -spec macx-xcode -o foo.xcodeproj bar.pro => foo.xcodeproj/foo.xcodeproj (wrong!)