Show warning for epsilon alts in left-recursive rules

[parrt]

this will cause weirdness.

e : e '*' e
  | e op
  | INT
  |
  ;

[sharwell]

This should actually work fine. The only "epsilon" problem in a left-recursive rule would be the epsilon closure produced by something like the following (which is covered by error 148 EPSILON_LR_FOLLOW):

e : e '*' e
  | e op
  | INT
  | e
  ;

[sharwell]

I updated the title - while the semantics of empty alternatives are clear and the behavior shouldn't be a problem for ANTLR 4, it's probably not a situation the user intended.

[parrt]

Sam: did you fix this for 4.1? I think we have an empty alternative checker but maybe not for left recursion.

[sharwell]

I didn't add the warning yet, but the code itself should work fine.

[parrt]

Meaning that it will strip out the epsilon without warning? Maybe I should put that warning in and we can close this.

[sharwell]

Why does it need to match anything? The epsilon stays it place, and allows it to match the empty string wherever you have an e reference (for the above case). Do you have a specific input for which the original rule you posted in the issue results in "weirdness"? Everything I thought of has a clear result that wouldn't cause problems.

[parrt]

I can't remember the weirdness now. no doubt it simply didn't properly translate. It still probably not what the user wants as it would allow input '************'. I guess we will leave this bug open until there is a warning generated