On 3941ab4, ({set x(a,) {}}) and ({set x({a},) {}}) parse and execute successfully, while system JavaScriptCore raises SyntaxError. The shared parser uses ordinary FormalParameters for object/class setters, then validates only the resulting parameter count/rest flag after parsing the body. A trailing comma disappears from that representation, so the count-only check cannot enforce PropertySetParameterList.
The Method Definitions grammar permits one FormalParameter, not an ordinary multi-parameter or trailing-comma list. Getter parameter errors are also diagnosed after parsing their bodies, which loses the offending token and can report a later body fault instead.
Plan: carry accessor kind into parameter parsing; reject illegal arity/rest/comma syntax at the actual token; preserve ordinary function/method trailing commas and valid setter defaults/destructuring. Verify object, class, static/private/computed accessors with parser and eval/Function tests, system JSC differential, and affected test262 subtrees. This is a syntax acceptance fix as well as a diagnostic improvement, not a string-only change.