The following fails to compile:

bool flag() {
        bool left, right;
        return true ? cast(const bool)left : cast(bool)right;
}

with the error "cannot implicitly convert expression (cast(int)left) of type int to bool".  It seems to happen whenever the left and right sides differ in const-ness or invariant-ness.

This becomes an issue when trying to write const-correct class functions which return bool, for example:

class C {
    private int mask;

    private bool readyYet() const {return true;}

    public bool ExtractStateBit() const {
        return readyYet() ? (mask & 1) : false;
    }
}