Note that this was running with a build made around 2011-02-10!

Given the class

public class Triangle {
public enum Type

{ INCOMPLETE, UNCLASSIFIED, EQUILATERAL, ISOSCELES, RECTANGLED, ISOSCELES_RECTANGLED, ACUTE, OBTUSE; }

private int alpha;
private int beta;
private int gamma;
private Type type;
public Triangle( int alpha, int beta )

{ this.alpha = alpha; this.beta = beta; this.type = Type.UNCLASSIFIED; }

// getters, setter,...
}

and the rule

rule equilateral
when
$t: Triangle( type == Triangle.Type.UNCLASSIFIED,
alpha == beta && == gamma )
then
modify( $t )

{ setType( Triangle.Type.EQUILATERAL ) }

end

this fails with

Exception in thread "main" org.drools.RuntimeDroolsException: [Error: object is not an instance of declaring class]
[Near :

{... Triangle.Type.UNCLASSIFIED ....}]
^
[Line: 1, Column: 1]
at org.drools.rule.ReturnValueRestriction.isAllowed(ReturnValueRestriction.java:258)
at org.drools.rule.VariableConstraint.isAllowed(VariableConstraint.java:103)
at org.drools.reteoo.AlphaNode.assertObject(AlphaNode.java:134)
at org.drools.reteoo.CompositeObjectSinkAdapter.doPropagateAssertObject(CompositeObjectSinkAdapter.java:450)
at org.drools.reteoo.CompositeObjectSinkAdapter.propagateAssertObject(CompositeObjectSinkAdapter.java:378)
at org.drools.reteoo.ObjectTypeNode.assertObject(ObjectTypeNode.java:185)
at org.drools.reteoo.EntryPointNode.assertObject(EntryPointNode.java:143)
at org.drools.common.NamedEntryPoint.insert(NamedEntryPoint.java:332)
at org.drools.common.NamedEntryPoint.insert(NamedEntryPoint.java:293)
at org.drools.common.AbstractWorkingMemory.insert(AbstractWorkingMemory.java:905)
at org.drools.common.AbstractWorkingMemory.insert(AbstractWorkingMemory.java:864)
at org.drools.impl.StatefulKnowledgeSessionImpl.insert(StatefulKnowledgeSessionImpl.java:251)
at rss.drools.setup.Application.insert(Application.java:141)
at ex4.Main.makeFacts(Main.java:35)
at rss.drools.setup.Application.execute(Application.java:83)
at ex4.Main.main(Main.java:40)
Caused by: [Error: object is not an instance of declaring class]
[Near : {... Triangle.Type.UNCLASSIFIED ....}

]
^
[Line: 1, Column: 1]
at org.mvel2.optimizers.impl.refl.ReflectiveAccessorOptimizer.compileGetChain(ReflectiveAccessorOptimizer.java:427)
at org.mvel2.optimizers.impl.refl.ReflectiveAccessorOptimizer.optimizeAccessor(ReflectiveAccessorOptimizer.java:140)
at org.mvel2.ast.ASTNode.optimize(ASTNode.java:158)
at org.mvel2.ast.ASTNode.getReducedValueAccelerated(ASTNode.java:115)
at org.mvel2.MVELRuntime.execute(MVELRuntime.java:87)
at org.mvel2.compiler.CompiledExpression.getValue(CompiledExpression.java:125)
at org.mvel2.compiler.CompiledExpression.getValue(CompiledExpression.java:118)
at org.mvel2.MVEL.executeExpression(MVEL.java:954)
at org.drools.base.mvel.MVELReturnValueExpression.evaluate(MVELReturnValueExpression.java:88)
at org.drools.rule.ReturnValueRestriction.isAllowed(ReturnValueRestriction.java:247)
... 15 more
Caused by: java.lang.IllegalArgumentException: object is not an instance of declaring class
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:616)
at org.mvel2.optimizers.impl.refl.ReflectiveAccessorOptimizer.getBeanProperty(ReflectiveAccessorOptimizer.java:514)
at org.mvel2.optimizers.impl.refl.ReflectiveAccessorOptimizer.compileGetChain(ReflectiveAccessorOptimizer.java:335)
... 24 more