Parse error on type assertion with contextual keyword in parentheses (override as Type)

[zxbodya]

Bug Report

🔎 Search Terms

typecast, parentheses, override

🕗 Version & Regression Information

Started to see the issue after updating from TypeScript 4.2 to 4.3, initially noticing this when running prettier after updating it prettier/prettier#11018

It also is happening in current 'Nightly' version in the playground.

⏯ Playground Link

Playground link with relevant code

💻 Code

const override = {};
const b = (override as any);

🙁 Actual behavior

Code results in a syntax error.

However it works fine when renaming variable override to something else.

🙂 Expected behavior

Code above should work

[ShuiRuTian]

The reason is override become a new keyword. This is something like you could not name the variable to public, function, const....
Because when scanning, tokenlizer would scan this word as XXXKeyword rather than Identifier.
So, this is a breaking change, rather than a bug?

[DanielRosenwasser]

@ShuiRuTian override is a contextual keyword, and TypeScript needs to support valid uses of it in JS.

From my comment at #44996 (comment)

Ugh - the same is true of (at least) readonly, declare, and abstract.

[ShuiRuTian]

@DanielRosenwasser interesting, if so , I want to have a try to fix this issue.

[DanielRosenwasser]

Go for it, though I don't know what precisely to do here.

Maybe when parsing a parameter for anything other than a constructor declaration, a keyword that isn't reserved in JS (such as override, readonly, declare, and abstract) should be re-scanned as a plain Identifier.

[DavidSouther]

In tryParseParenthesizedArrowFunctionExpression, the lookahead treats this as OpenParen Modifier Identifier, when it should not expect as to be an identifier. I'm not sure if this is the correct or most robust fix, but it feels heuristically right for this and attached bugs, and I can't think of other parenthesized expressions that use keywords in this way. If there are, this should provide a good jumping off point to handle those in isolation, but this is my first time poking this parser :)