I think the correctness of adding multiple of the same label ("id") is non-obvious here and it would be worth it to explain it more...

You somewhat described it in 1547, but I don't understand how exactly duplicate enum values would be OK, precisely?

Like

enum invocation_label {
    InvalidInvocation,
    {%- for label, condition in invocations %}
    {%- if condition %}
#if {{condition}}
    {%- endif %}
    {{label}},
    {%- if condition %}
#endif
    {%- endif %}
    {%- endfor %}
    nInvocationLabels
};

would produce an enum like?

enum invocation_label {
   Invalid,
   LabelOne,
   LabelOne,
   // etc
}

I don't understand how that works, unless the conditions are disjoint?

I can't remember whether I needed to add that to make it work, or that it fixes a theoretical problem. I suspect it's the former, but not sure.

It handles the case where there are multiple labels with the same name, but different conditions. Of course, if #1542 gets implemented, this won't be needed any more.

Example:

enum invocation_label {
   Invalid,
#if A
   LabelOne,
#endif
#if B
   LabelOne,
#endif
   // etc
}