Segfault with ill-formed tuple-based loops

[asutton]

This causes an ICE:

$class metaclass {
    constexpr {
        for (auto func : $metaclass.member_functions()) {
            // TODO: something interesting here..
        } 
    }
};

The error occurs in EvaluateStatement, which means that the loop is not being properly checked during transformation or substitution (tuples aren't ranges, so we should get a compiler error).

[asutton]

Not actually fixed. Just improved the diagnostics outside of the given example.