Generated internal parser contains java syntax error

I'm seeing a syntax error in the generated InternalMyDslParser.java file:

        // InternalMyDsl.g:139:7: ( ( ( ',' ( ( ruleSwitchCaseOrDefault ) ) ) )=> ( ',' ( ( ruleSwitchCaseOrDefault ) ) ) )*
        loop:
        do {
            int alt=;  // SEE THIS LINE
            switch (alt) {
        	case 1 :
        	    ...
        	    break;
        	default :
        	    break loop;
            }
        } while (true);

Minimal repro grammar file:

grammar org.xtext.example.mydsl.MyDsl with org.eclipse.xtext.common.Terminals

generate myDsl "http://www.xtext.org/example/mydsl/MyDsl"

Expression:
	expression=SwitchOperation;

SwitchOperation returns Expression:
	Integer
	(
		=>({SwitchOperation.argument=current} operator='switch' =>(cases+=SwitchCaseOrDefault =>(',' cases+=SwitchCaseOrDefault)*)?)
	)*
;

SwitchCaseOrDefault:
	'default' expression=Expression
 	| guard=Integer 'then' expression=Expression
;

Integer returns Expression:
	value=INT
;

Is there any workaround for this?

Start with the minimal repro grammar and regenerate InternalMyDslParser.java, confirming the invalid int alt=; output. Trace the parser-generation path responsible for the switch loop and consider the issue complete when the generated Java is syntactically valid for this grammar and the reproduction no longer fails.