* Properly handle elements that are optional in some alts but not others
* Properly handle block sets (a group of terminals producing a SetTransition)
* Properly handle OPTIONAL subrule
These tests were modeled after the listener tests, but were not implemented
against the standard visitor API semantics. The tests only execute in the
Node.js target, and there is no straightforward way to implement these tests
across multiple targets. New tests which cover both the traversal behavior
and the runtime semantics are now available and should be implemented by
each target.
This analysis is required for proper code generation in the TypeScript target
when strict null checks are enabled. It also applies to targets intending to
differentiate optional values from required values.
```
beast:/tmp $ a4.6 T.g4
org/antlr/v4/parse/GrammarTreeVisitor.g: node from line 2:7 no viable alternative at input '..'
org/antlr/v4/parse/GrammarTreeVisitor.g: node from line 2:7 no viable alternative at input '..'
org/antlr/v4/parse/GrammarTreeVisitor.g: node from line 2:7 no viable alternative at input '..'
org/antlr/v4/parse/GrammarTreeVisitor.g: node from line 2:7 no viable alternative at input '..'
org/antlr/v4/parse/GrammarTreeVisitor.g: node from line 2:7 no viable alternative at input '..'
context [/report INTERNAL_ERROR] 1:17 attribute arg isn't defined
error(20): internal error:
beast:/tmp $ a4.6.1 T.g4
error(181): T.g4:2:4: token ranges not allowed in parser: 'A'..'Z'
```
We know by the time this check was reached that neither EPSILON nor the LL(1)
symbol are in the lookahead set from the current state. Since EPSILON is not
included, the state cannot see to the end of the rule and thus nextTokens
contains the complete set of valid LL(1) symbols from the current state. It is
therefore impossible for the LL(1) symbol to be "expected" when considering
lookahead with full context.
This change updates the default sync() strategy to match the strategy used
for selecting an alternative when prediction leaves the decision rule prior
to reaching a syntax error.
Closes#1545
Terence Parr
Sam Harwell
Eric Vergnaud
Mike Lischke
Ivan Kochurkin
Adrian Pop
Dong Xie
Akos Kiss
Mike Lischke