Correct handling of newline after async with paren-less arrow func

[Retsam]

Q	A
Fixed Issues?	Fixes #8795
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

Assuming the issue reported in #8795 is correct (I haven't read the spec, but the described behavior seems to match other parsers such as flow or typescript), then

async
x => x

should parse as two expression statements, not a single arrow function like it currently does.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/9221/

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/9221/

[nicolo-ribaudo]

async
(x) => {}

Already works?

[Retsam]

async
(x) => x

is a parse error, which seems consistent how other parsers treat it. (e.g. acorn) I'm not good enough at reading the spec to say if that's the correct behavior.

[nicolo-ribaudo]

I think that it should be allowed (they are two expression statements), but you can test it in the console of a modern browser.

[Retsam]

Yeah, the version with parens is a syntax error in Firefox, Chrome, and Safari. (And the paren-less version is not)

[nicolo-ribaudo]

Oh ok 👍

[pvdz]

Yeah the spec is a little difficult to read. Basically you're supposed to parse async( as a regular call first, in which case the newline between async and ( is fine. Then if you find an arrow, you're supposed to apply the cover grammar, which disallows the newline. If the cover grammar can't be applied the parser must consider it a syntax error.

For the paren-less case there is no cover grammar to apply as there is no ambiguity. If there's a newline there's an ASI and if there's no newline then async foo can only lead to an arrow (well, or a crash :) )

Thanks for fixing :)