transform-es2015-parameters: arguments deoptimized when shadowed in nested function

[jpommerening]

This is a feature request (I think). Arguments are not optimized if an inner function shadows the name with a parameter (or rest parameters in my case).

Input Code

const log = (...args) => console.log(...args);

function test_opt(...args) {
  log(...args);
}

function test_deopt(...args) {
  const fn = (...args) => log(...args);
  fn(...args);
}

Babel Configuration (.babelrc, package.json, cli command)

{
  "presets": [ "es2015" ]
}

Expected vs. Current Behavior

I'd expect the code to be optimizable to use .apply( thisArg, arguments ) throughout.
However, in test_deopt the outer ...args gets copied just to be passed into the inner fn.
I can verify that the problem disappears if I rename either the ...args of test_deopt or of the fn arrow function.

Environment

software	version(s)
Babel	6.24.1
node	6.9.5
npm	N/A (yarn 0.20.3)
Operating System	macOS

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[Qantas94Heavy]

I hope I'm not being stupid, but is this really meant to be "expected"?

Test code:

var y = function (foo, ...bar) {
  var x = function z(bar) {
    bar[1] = 5;
  };
};

Current "expected" result:

var y = function (foo) {
  for (var _len2 = arguments.length, bar = Array(_len2 > 1 ? _len2 - 1 : 0), _key2 = 1; _key2 < _len2; _key2++) {
    bar[_key2 - 1] = arguments[_key2];
  }

  var x = function z(bar) {
    bar[1] = 5;
  };
};

I would have thought that given the inner bar shadows the outer rest bar, we wouldn't need to copy the arguments.

[jpommerening]

Hey @Qantas94Heavy, thanks for pointing that out!
I was under the impression that this was merely an oversight / missing optimization. I didn't know there was actually a test verifying the (arguably) incorrect behavior. I'm as confused as you are that the transform is expected to copy the arguments in the test case you linked.