Semantic predicates prevent fallback to full-context parsing

[sharwell]

Currently full-context parsing is prevented when one or more configs contain semantic predicates. However, full-context parsing may eliminate one or more configs before the predicate evaluation stage which could change the predicted alternative.

[parrt]

If we have 3 alts with predicates and they are all ambig in local context but alt 1 is eliminated in full ctx parse. but, we choose first alt with local ctx. we don't check full ctx parse first. Not sure what to do. maybe just doc it ;)

[sharwell]

Local/full context handling is independent of semantic context handling. If the presence of semantic contexts prevents the fallback to full context parsing, then using an LR rule in a grammar could turn an otherwise LL() parser into an SLL() parser, undoing one of ANTLR 4's most significant contributions.

[sharwell]

This issue is simply not correct. The parser does fall back to full context parsing even when semantic predicates are present.