Nested object rest in destructuring results in code with a syntax error

[evanw]

Bug Report

🔎 Search Terms

array object assign destructuring invalid code syntax error

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

Playground link with relevant code

💻 Code

var x, y, a, b;

// Should print: {"abc":1}
[{ ...x }] = [{ abc: 1 }]; console.log(JSON.stringify(x));

// Should print: {"abc":1}
for ([{ ...y }] of [[{ abc: 1 }]]) console.log(JSON.stringify(y));

// Should print: {"a":"a","b":{"1":"b","2":"c"}}
for ({ 0: a, ...b } in { abc: 1 }) console.log(JSON.stringify({a, b}));

🙁 Actual behavior

Compiling this with a target of ES2017 results in code with a syntax error:

"use strict";
var x, y, a, b;
// Should print: {"abc":1}
[Object.assign({}, x)] = [{ abc: 1 }];
console.log(JSON.stringify(x));
// Should print: {"abc":1}
for ([Object.assign({}, y)] of [[{ abc: 1 }]])
    console.log(JSON.stringify(y));
// Should print: {"a":"a","b":{"1":"b","2":"c"}}
for (Object.assign({ 0: a }, b) in { abc: 1 })
    console.log(JSON.stringify({ a, b }));

The specific syntax error is SyntaxError: Invalid destructuring assignment target, and happens because of the call to Object.assign in assignment position.

🙂 Expected behavior

This should result in valid code, and that code should print this:

{"abc":1}
{"abc":1}
{"a":"a","b":{"1":"b","2":"c"}}

I have reported this same issue in the past: #39181 (comment). It must be that either the fix has regressed or the fix was incomplete.

[a-tarasyuk]

TS doesn't allow to use destructuring in for...in statement. @RyanCavanaugh Do we need to allow it?

for ({ 0: a, ...b } in { abc: 1 }) console.log(JSON.stringify({a, b}));
// The left-hand side of a 'for...in' statement cannot be a destructuring pattern.(2491)