Brief summary of the change made

Fixes #6784.

Problem

SQLFluff couldn't parse PostgreSQL function calls with VARIADIC keyword, throwing "unparseable section" errors. The issue example was:

SELECT range_merge(datemultirange(
    VARIADIC ARRAY_REMOVE(ARRAY[
        a.source_date,
        b.source_date,
        c.source_date
    ], NULL)
))::daterange
FROM a
FULL OUTER JOIN b USING (h3_index)
FULL OUTER JOIN c USING (h3_index)

Before this fix

$ echo "SELECT range_merge(datemultirange(
    VARIADIC ARRAY_REMOVE(ARRAY[
        a.source_date,
        b.source_date,
        c.source_date
    ], NULL)
))::daterange
FROM a
FULL OUTER JOIN b USING (h3_index)
FULL OUTER JOIN c USING (h3_index);" | sqlfluff parse - --dialect postgres
[L:  1, P:  1]      |file:
[L:  1, P:  1]      |    statement:
[L:  1, P:  1]      |        select_statement:
...
[L: 10, P: 35]      |    statement_terminator:                                     ';'
[L: 10, P: 36]      |    newline:                                                  '\n'
[L: 11, P:  1]      |    [META] end_of_file:

==== parsing violations ====
L:   2 | P:   5 |  PRS | Line 2, Position 5: Found unparsable section: 'VARIADIC
                       | ARRAY_REMOVE(ARRAY[\n        a.s...'
WARNING: Parsing errors found and dialect is set to 'postgres'. Have you configured your dialect correctly?

What I did

Added Sequence("VARIADIC", Ref("ExpressionSegment")) to the PostgreSQL FunctionContentsGrammar. Pretty simple fix that now handles:

• VARIADIC ARRAY[1, 2, 3]
• VARIADIC ARRAY_REMOVE(...)
• VARIADIC ARRAY(SELECT ...)
• VARIADIC my_array

Why this approach

Went with Sequence("VARIADIC", Ref("ExpressionSegment")) because:

• PostgreSQL's VARIADIC syntax is always function_name(VARIADIC expression) where the expression can be arrays, function calls, variables, or subqueries
• VARIADIC isn't optional - you either use it or you don't, no in-between
• Added it to the AnyNumberOf in FunctionContentsGrammar so it plays nice with other function arguments
• Using ExpressionSegment covers all the different things that can come after VARIADIC without needing separate rules for each

After this fix

$ echo "SELECT range_merge(datemultirange(
    VARIADIC ARRAY_REMOVE(ARRAY[
        a.source_date,
        b.source_date,
        c.source_date
    ], NULL)
))::daterange
FROM a
FULL OUTER JOIN b USING (h3_index)
FULL OUTER JOIN c USING (h3_index);" | sqlfluff parse - --dialect postgres
[L:  1, P:  1]      |file:
[L:  1, P:  1]      |    statement:
[L:  1, P:  1]      |        select_statement:
...
[L: 10, P: 35]      |    statement_terminator:                                     ';'
[L: 10, P: 36]      |    newline:                                                  '\n'
[L: 11, P:  1]      |    [META] end_of_file:

Are there any other side effects of this change that we should be aware of?

Nope, no breaking changes. Just adds support for something that should have worked already. Only affects PostgreSQL dialect.

Pull Request checklist

• Please confirm you have completed any of the necessary steps below.
• Included test cases to demonstrate any code changes, which may be one or more of the following:
  • .yml rule test cases in test/fixtures/rules/std_rule_cases.
  • .sql/.yml parser test cases in test/fixtures/dialects (note YML files can be auto generated with tox -e generate-fixture-yml).
  • Full autofix test cases in test/fixtures/linter/autofix.
  • Other.
• Added appropriate documentation for the change.
• Created GitHub issues for any relevant followup/future enhancements if appropriate.