ANTLR4 Mutual left recursion

Question

I just ran into a strange problem with ANTLR 4.2.2:

Consider a (simplified) java grammar. This does not compile:

 classOrInterfaceType
    :  (classOrInterfaceType) '.' Identifier
    |  Identifier 
    ;

ANTLR outputs the following error:

error(119): Java.g4::: The following sets of rules are mutually left-recursive [classOrInterfaceType]

Yes, I also see a left recursion. But I do not see a mutual left recursion, only a usual one.

When I remove the parenthesis around (classOrInterfaceType), then it compiles fine. Of course, the parenthesis are superfluous, but the grammar is generated automatically and the code generator always inserts parenthesis in some situations. So what is the problem here?

Answer 1

It has been confirmed that this is a bug. The fix is scheduled for the next milestone 4.x. See https://github.com/antlr/antlr4/issues/564