ASI not working after arguments-less new expression with type arguments

[nicolo-ribaudo]

Bug Report

🔎 Search Terms

asi new expression type arguments

🕗 Version & Regression Information

TypeScript 4.7 Nightly (tested with the playground). This syntax has never been valid before TS 4.7.

new X<T>; is valid syntax since #47607 (cc @ahejlsberg)

⏯ Playground Link

Playground link with relevant code

💻 Code

class A<T> {}

new A<number>

if (0) {}

🙁 Actual behavior

It's parsed as if it was

class A<T> {}

new A<number> if (0) {}

thus it reports a few errors (number is used in a value position, and if is a reserved word).

🙂 Expected behavior

I would expect ASI to kick in, and to parse the code as if it was

class A<T> {}

new A<number>;

if (0) {}

Context

I'm implementing this in Babel. If it's expected that ASI does not work here, what are the rules to differentiate between a new expression with only type arguments, and a relational comparison?

[nicolo-ribaudo]

Fwiw, the current implementation I'm doing in Babel (unless this is confirmed to be the expected behavior) is to bail out from parsing A <T> as an expression with type arguments if it's followed by a non-( token that can appear at the beginning of an expression.

[ahejlsberg]

Yeah, we're probably being a bit too conservative here. Let me think about it and get back...

[ahejlsberg]

I agree we can be a bit more permissive here. I think we should consider an expression of the form T<X> to be an expression with type arguments if it is followed by a ( token, a NoSubstitutionTemplate production, a TemplateHead production, or any token that can't start an expression. So, basically the rule you suggested, but also allowing for generic tagged template literals.

[nicolo-ribaudo]

@ahejlsberg I don't know if it can make sense semantically, but I'm also surprised that this is not parsed as an instantiation expression:

b<x> ?? 2;