This doesn't seem right. The error position is reported as the end of UNBOUNDED:

SELECT X() OVER (ROWS UNBOUNDED) FROM T
                               ^

The only valid tokens at that point are FOLLOWING and PRECEDING, so we shouldn't be reporting all of these extra tokens.

Yes, I know. I’m trying to figure out why it’s doing that. There’s some other bug lurking in there.

It turns out these are all valid tokens per the grammar. It's because UNBOUNDED is a non-reserved word, so it can take the place of an identifier. In particular, it's considering alternatives that include the following productions:

over
    : OVER ([...] | '(' windowSpecification ')')
    ;

windowSpecification
    : [...]
      windowFrame?
    ;

windowFrame
    : [...]
      frameExtent
    ;

frameExtent
    : [...]
    | frameType=ROWS start=frameBound
    | [...]
    ;

frameBound
    : [...]
    | expression boundType=(PRECEDING | FOLLOWING)  #boundedFrame
    ;

And <expression> can be an <identifier> followed by a number of things, including ., [, OVER, etc.

For instance, this goes through the parser successfully even though it's clearly wrong if we consider UNBOUNDED a special keyword:

SELECT X() OVER (ROWS UNBOUNDED -> 1 PRECEDING) FROM t

It's treating UNBOUNDED -> 1 as a lambda expression, which is a valid (syntax-wise) expression in the "#boundedFrame" frame bound above.

If I mark UNBOUNDED as a reserved word, then it produces the expected message:

line 1:32: mismatched input ')'. Expecting: 'FOLLOWING', 'PRECEDING'

Is there a way to make the error handler treat UNBOUNDED as a reserved word without actually making it reserved? In my opinion, printing all alternatives for the case of UNBOUNDED being an identifier is unnecessary and confusing, even if those are all correct per grammar.

Another issue I can spot here is that the error handler suggests GROUPS, RANGE, and ROWS at position 23:

Arguments.of("SELECT x() over (ROWS select) FROM t",
                        "line 1:23: mismatched input 'select'. Expecting: 'BETWEEN', 'CURRENT', 'UNBOUNDED', <expression>"),
                        "line 1:23: mismatched input 'select'. Expecting: ')', 'BETWEEN', 'CURRENT', 'GROUPS', 'MEASURES', 'ORDER', 'PARTITION', 'RANGE', 'ROWS', 'UNBOUNDED', <expression>"),

It is not correct, as position 23 is after the word ROWS. It seems that the reported error position is the innermost one, but the suggestions come from an outer rule.

It is correct (grammar-wise) because ROWS can be interpreted as an identifier representing a named window. After that, any of the terms PARTITION, ORDER, or any of the prefix tokens in windowFrame are valid.

At the end of the day, this fixes a bug in the parser error handler that should've never existed in the initial implementation, so we should get it in. I can't think of any way to avoid the spurious tokens due to the non-reserved keyword handling without adding semantic knowledge to the grammar about the behavior of each rule, but I have some ideas that are out of the scope of this PR.

Do you think we can remove ignoredRule from ErrorHandler now?
AFAIK, nonReserved was the only ignored rule.

Yes, we can remove that.