• Read dialect Trino
• Write dialect Trino
• Still exists in main

Fully reproducible code snippet

MERGE INTO table_a AS target USING(
  SELECT
    pk,
    my_array
  FROM table_b
) AS source ON source.pk = target.pk
WHEN MATCHED THEN UPDATE SET target.my_array = ARRAY_DISTINCT(CONCAT(source.my_array, target.my_array))
WHEN NOT MATCHED THEN INSERT (pk, my_array)
VALUES (
  source.pk,
  source.my_array
)

As the WHEN MATCHED is a function then this should be possible but there's a transformation applied that strips out the table aliases in the THEN part.
This then leads us to an ambiguous column error as target.my_array is transformed to just my_array.

Might a solution be to also consider if the column's parent is a function? In which case if the merge target is referenced then it is to remove ambiguity.
I'll raise a PR with such a solution shortly and will await comments.

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