DuckDB: f(a == b) is displayed as f(a = b), which reparses as a named argument #2808

== is parsed as BinaryOperator::Eq and displayed as =. Inside a function call with DuckDbDialect, a = b is a named argument (supports_named_fn_args_with_eq_operator, #1182), so the output means something else:

SELECT coalesce(a == b) FROM t
  -> AST arg: Unnamed(BinaryOp { a Eq b })
  -> displays as: SELECT coalesce(a = b) FROM t
  -> reparse arg: Named { name: a, arg: b, operator: Equals }
SELECT a == b FROM t
  -> displays as SELECT a = b FROM t, same AST (fine)

DuckDB documents = and == as equal (https://duckdb.org/docs/stable/sql/expressions/comparison_operators) and evaluates coalesce(a == b) as a comparison. This may count as accepted normalisation; the only effect is the AST change inside call arguments.

Tested on main ce12f1f (also 0.63.0 and 0.58.0).

Found with a property-based fuzzer (round trip check); I used AI assistance for triage and verified every case above.