@babel/generator comment deduplication removes too many comments

[mischnic]

Bug Report

• I would like to work on a fix!

Input Code

import { parse } from "@babel/parser";
import generator from "@babel/generator";
import t from "@babel/types";

const code1 = `
// hello!
a();
`;

const code2 = `
// hello
b();
`;

let ast1 = parse(code1).program;
let ast2 = parse(code2).program;

let ast = t.program([...ast1.body, ...ast2.body]);

console.log(generator.default(ast).code);

Expected behavior

// hello!
a();
// hello
b();

Current behavior

// hello!
a();
b();

Environment

    "@babel/generator": "7.12.15",
    "@babel/parser": "7.12.15",

Possible Solution

Additional context

I was surprised that there are cases where comment exists twice (as leading and trailing), e.g.

if (1) {
  a();
}
// /* */
else {
  b();
}

. Looks like babel-generators "workaround" for this isn't quite right.

Distantly related to parcel-bundler/parcel#5787

[babel-bot]

Hey @mischnic! We really appreciate you taking the time to report an issue. The collaborators on this project attempt to help as many people as possible, but we're a limited number of volunteers, so it's possible this won't be addressed swiftly.

If you need any help, or just have general Babel or JavaScript questions, we have a vibrant Slack community that typically always has someone willing to help. You can sign-up here for an invite."

[kaicataldo]

This does seem like unexpected/destructive behavior to me, though I think the example given in #13169 (comment) is a more realistic repro case. Curious what the rest of the team thinks.

[kaicataldo]

I don't think this is fixed yet since we split #13169 into two separate PRs. Reopening for now.