Generated code is incorrect for function with destructuring and default parameter

[evanw]

Bug Report

🔎 Search Terms

default destructuring scope parameter argument ReferenceError

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

Playground link with relevant code

💻 Code

function foo({a, ...x}, b = a) {
  return b
}
console.log(foo({a: 1}))

🙁 Actual behavior

If you run TypeScript's output, you get ReferenceError: a is not defined. This is the problematic output:

"use strict";
var __rest = (this && this.__rest) || function (s, e) {
    var t = {};
    for (var p in s) if (Object.prototype.hasOwnProperty.call(s, p) && e.indexOf(p) < 0)
        t[p] = s[p];
    if (s != null && typeof Object.getOwnPropertySymbols === "function")
        for (var i = 0, p = Object.getOwnPropertySymbols(s); i < p.length; i++) {
            if (e.indexOf(p[i]) < 0 && Object.prototype.propertyIsEnumerable.call(s, p[i]))
                t[p[i]] = s[p[i]];
        }
    return t;
};
function foo(_a, b = a) {
    var { a } = _a, x = __rest(_a, ["a"]);
    return b;
}
console.log(foo({ a: 1 }));

Notice how b = a is evaluated before { a } = _a.

🙂 Expected behavior

If you run the JavaScript directly you should get 1, so TypeScript's output should do that too.

Additional context is here: evanw/esbuild#1826. I got a report about this bug in esbuild and it turns out TypeScript has the same bug, so I'm filing it here too. My apologies if this is a duplicate (I couldn't find one that had already been filed).

[RyanCavanaugh]

Appreciate the report as usual @evanw 👍