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If you track through the grammar, you get (removing paths irrelevant to this problem): InterfaceMember → AttributeOrOperationOrIterator AttributeOrOperationOrIterator → Qualifier AttributeOrOperationRest Qualifier → "static" | "stringifier" AttributeOrOperationRest → OperationRest OperationRest → OptionalIdentifier "(" ArgumentList ")" ";" So putting all of that together, it's impossible to specify a return type for static or stringifier operations. It should probably be ReturnType OperationRest in there.
*** This bug has been marked as a duplicate of bug 18424 ***