If I give an ID to say, the cpp module in GenericGCC.qbs, and then reference a property from a Probe in that module, for example:

// GenericGCC.qbs
import qbs
Module {
    id: genericGCC
    Probe {
        toolchainInstallPath: genericGCC.toolchainInstallPath
    }
}

...I get asserts:

SOFT ASSERT: varValue in /Users/jakepetroules/OpenSource/qbs/qbs/src/lib/corelib/language/evaluatorscriptclass.cpp:576

Everything still seems to work though. If I define the same ID in the derived item (say, windows-mingw.qbs), the asserts go away.