Lookahead tokens don't understand optional prefixes

[MichaelRFairhurst]

%lookahead flag x;

prog: optionallyPrefixed<+x>;
optionallyPrefixed: prefix? usesLookahead ;
usesLookahead: [x] y | z;

This reported that x was unused in optionallyPrefixed, until I changed that production to

optionallyPrefixed: usesLookahead | prefix usesLookahead ;

[inspirer]

Yes, template instantiation happens before EBNF expansion, and this is rather non-trivial to combine in one step. I'll add better error reporting though. Another input that does not seem to trigger any errors, while it should, is when optionallyPrefixed starts with a nullable nonterminal.

[MichaelRFairhurst]

Ah, hadn't thought about epsilon. Great catch! And totally, the workaround is easy if it reports an error.