Creating a setter method that takes extra arguments in Ruby
It doesn't work because the parser doesn't allow it. An equals sign is allowed in expressions of the form identifier = expression, expression.identifier = expression (where identifier is \w+), expression[arguments] = expression and expression.[]= arguments and as part of a string or symbol or character literal (?=). That's it.
gen.send(:allow=, 'a', 1, false) would work, but at that point you could as well just give the method a name that doesn't include a =.