Crash when applying overloading to subsort productions that do not have the symbol(_) attribute

[Baltoli]

When working on #4045, I noticed that there was one case where a direct translation of klabel(X) → overload(X) did not behave as we expected it to. This is because we missed a behaviour of klabel(X) when originally specifying the overload(_) migration.

That behaviour is that klabel(X) will create an internal¹ klabel for productions that would not otherwise have one. The implementation of overloading in ModuleToKORE requires implicitly² that overloaded symbols have an internal klabel associated with them. Under the previous implementation of overloading, it was not possible to generate an overloaded symbol without a klabel, but the new implementation permits this.

To fix the issue, we should add a check that overloaded subsorting productions have an associated symbol(_) attribute so that they have an internal klabel during KORE generation.

Footnotes

1. These internal klabels are different to the attribute klabel(_). Roughly speaking, productions that have an internal klabel will be generated as KORE symbols, and those that don't will not. For instance, subsorting productions will not usually be KORE symbols. ↩

2. i.e. it crashes when this assumption is violated. ↩