Fix printing of single-param async arrow function with comments

[nwalters512]

Q	A
Fixed Issues?	Fixes #12500
Patch: Bug Fix?	Yes
Major: Breaking Change?	No
Minor: New Feature?	No
Tests Added + Pass?	Yes
Any Dependency Changes?	No
License	MIT

This PR ensures that the parameter of a single-parameter async arrow function is printed inside parentheses if the parameter has leading or trailing comments. Without the parentheses, the generated code is not interpreted correctly and results in a syntax error. See #12500 for the original report of this problem.

It's not clear to me why so much care went into ensuring that single-argument functions don't include unnecessary parentheses - this code could be simplified and made much more robust if we were willing to unconditionally add parentheses around all arrow function parameters. I'd be happy to make that change, but I figured I'd check in with the maintainers first in case there's a good reason that parentheses are omitted if possible.

I also added support for an OVERWRITE environment variable for the babel-generator tests - the instructions in #12500 (comment) said that this should work, but support was missing from this particular test suite. Happy to pull this into a separate PR if requested.

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[nicolo-ribaudo]

Thanks!

Moving the OVERWRITE thing in a separate PR allows us listing it in the changelog as an Internal change, but imo it's ok to keep it here and not list it.

[nicolo-ribaudo]

It's not clear to me why so much care went into ensuring that single-argument functions don't include unnecessary parentheses - this code could be simplified and made much more robust if we were willing to unconditionally add parentheses around all arrow function parameters. I'd be happy to make that change, but I figured I'd check in with the maintainers first in case there's a good reason that parentheses are omitted if possible.

If we find another parens-related bug we can decide to stop using this logic and always printing them. That code has been like that for so long that I have no idea why we initially didn't always print the parens 🤷

[hzoo]

yeah agreed with @nicolo-ribaudo, simplifying it sounds good to me.

I think the reason why we probably did it is if you wanted to save bytes (though it would be covered by the minifier). We also removed a lot of other options from generator (quotes/tabs, etc) since it's not the focus and can replace it with recast/prettier for codemods instead.

[nwalters512]

@hzoo would you rather wait til another bug is found like @nicolo-ribaudo suggested? Otherwise, I can make that change now while the context is still fresh in my head.