`object-rest-spread` does not transpile correctly when binding is referenced in param initializer

Bug Report

• I would like to work on a fix!

Input Code

• REPL

({ a, ...O }, b = O, c = a) => {}

Current Behavior The code above is transformed to

(_ref, b = O, c = a) => {
  let {
    a
  } = _ref,
      O = babelHelpers.objectWithoutProperties(_ref, ["a"]);
};

where initializers b = O and c = a no longer refers to the destructured params.

Expected behavior/code It should transformed to

(function (_ref) {
  let { a } = _ref,
      O = babelHelpers.objectWithoutProperties(_ref, ["a"]);
  var b = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : O;
  var c = arguments.length > 2 && arguments[2] !== undefined ? arguments[2] : a;
});

Babel Configuration (babel.config.js, .babelrc, package.json#babel, cli command, .eslintrc)

• Filename: babel.config.js

{
  "plugins": { "@babel/proposal-object-rest-spread" }
}

Environment: REPL

Possible Solution I haven’t come up with a simple solution. Suggestions are welcome.

Solution 1: Fall back to transform-parameters when such pattern is detected

i) A function param declarator contains a RestElement. ii) The declarator is also referenced by another params’ initializers.

This solution will downgrade unnecessary codes, i.e.

({ a, ...O }, b = 2, c = O) => {}

will be transformed to

(function (_ref) {
  var a = _ref.a,
      O = babelHelpers.objectWithoutProperties(_ref, ["a"]);
  var b = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : 2;
  var c = arguments.length > 2 && arguments[2] !== undefined ? arguments[2] : O;
});

even when b = 2 is supported on the target platform.

Solution 2: Extract convertSingleParam from convertParams defined in transform-parameters, apply convertSingleParam if one of params satisfies the pattern mentioned above.

This solution should prevent downgrading the code as much as possible. i.e.

({ a, ...O }, b = 2, c = O) => {}

should be transformed to

(function (_ref, b = 2) {
 let { a } = _ref,
      O = babelHelpers.objectWithoutProperties(_ref, ["a"]);
   var c = arguments.length > 2 && arguments[2] !== undefined ? arguments[2] : O;
});

Additional context/Screenshots This issue is discovered when I am investigating #11278.

[vedantroy]

This issue seems interesting, I’ll try fixing it.

@TrejGun I think the 0 is actually a capital-O. Maybe a different letter should be used, to make it less confusing.

[TrejGun]

i mean part {a, ...0} ,0 (zero) a not valid name for variable. in other words you can’t do const 0 = 0 why it should be possible with destruction?