[drools] Extend enum declarations to support other declared enums

[drccrd]

declare enum cannot reference another declared enum constant as a field type / constructor arg

Summary

A DRL declare enum cannot use another declared enum as the type of one of its fields, and cannot use an enum literal (e.g. Foo.BAR) as a constructor argument. Both the legacy DRL compile path and the executable-model path fail, in different ways. This forces users to fall back to String lookup codes throughout cross-referenced enum sets, losing type safety.

Historical context

The limitation has existed since the declare enum feature was introduced. JBRULES-3009 commit ba47bd6 (2012-01-17) added the feature and added the canonical test fixture test_Enums.drl with the cross-enum case already commented out — the original author possibly intended this to work, wrote the test for it, then commented it out before merging. The lines have sat untouched for ~14 years.

Reproduction

package org.example;

declare enum Categories
  Target("target"),
  NonTarget("non-target"),
  NewLesion("new");

  code: String;
end

declare enum LesionStates
  TargetNotAssessed( org.example.Categories.Target, "notassessed" ),
  TargetPresent(     org.example.Categories.Target, "present"     );

  category: org.example.Categories;
  nominalAnswerType: String;
end

Compiling this DRL fails. The same shape appears commented-out in the canonical test fixture:

drools-test-coverage/test-compiler-integration/src/test/resources/org/drools/mvel/integrationtests/test_Enums.drl#L46-L54:

declare enum WorkLoad
//    LIGHT( x.DaysOfWeek.TUESDAY, 4 ),
//    MEDIUM( x.DaysOfWeek.MONDAY, 8 ),
//    HEAVY( x.DaysOfWeek.SUNDAY, 12 );

    LIGHT( 4 ), MEDIUM( 8 ), HEAVY( 12 );

//    day     : DaysOfWeek
    hours   : int
end

The commented-out lines are exactly the case that should work — strongly suggesting the limitation is known.