Parsing error for for (await using of of of) {}

[sosukesuzuki]

🔎 Search Terms

"using of"

🕗 Version & Regression Information

• This changed between versions 5.1 and 5.2_

⏯ Playground Link

https://www.typescriptlang.org/play?#code/N4KABGBmD2BOYAoCGB3JBLALmArgZ3QDsBzMaSMi8gSgG5wo4xUNtk0tcCTLeb6AviCA

💻 Code

{
  for (await using of of of);
}

🙁 Actual behavior

Parsing error The left-hand side of a 'for...of' statement must be a variable or a property access.

🙂 Expected behavior

Parse successful.

Additional information about the issue

Context: https://github.com/prettier/prettier/pull/15321/files#r1308180336
Spec: https://tc39.es/proposal-explicit-resource-management/#sec-for-in-and-for-of-statements

Babel can parse for (await using of of of) {}. ref: https://github.com/babel/babel/blob/b804bb08912d262133213dcd7f40d81645f97489/packages/babel-parser/test/fixtures/experimental/async-explicit-resource-management/valid-for-using-binding-of-of/input.js

[MartinJohns]

Does this ever happen with real code, or is it just an attempt to intentionally break the parser?

[sosukesuzuki]

I didn't see this with real code but I didn't intentionally break the parser, I just noticed it while developing the code formatter.

[DanielRosenwasser]

(congrats on issue #55555!)

[rbuckton]

Does this ever happen with real code, or is it just an attempt to intentionally break the parser?

It's a spec conformance issue. The spec specifically forbids for (using of of since that can be ambiguous (using could be an identifier like x in for (x of of, or of could be either an identifier or a keyword), while for (await using of of is unambiguous (await using can only imply the declaration, thus await using of must declare an identifier).