[mtlmtlmtlmtl]

I assume that an invalid program would not compile/parse, and so would die and fail to reproduce. The issue is more that if the space of invalid programs is too large compared to the space of valid ones, generating valid offspring by combining two programs would be too rare and the population would die off.

Though if the space is small enough I imagine you could get past that. It's a bit of a gnarly point, hard to tell how this would turn out without trying I suppose.

As for the halting problem there's of course no clever solution there other than limiting CPU time. So I guess pick a reasonable limit that makes sense for whatever you're trying to do.