fix: use async config loading if available

[the-spyke]

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• Tests for the changes have been added (for bug fixes / features)
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What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
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What is the current behavior? (You can also link to an open issue here)

Fixes #824

What is the new behavior?

If @babel/core is >=7.8.0, then ESM config will be found and loaded asynchronously.

Does this PR introduce a breaking change?

• Yes
• No

If this PR contains a breaking change, please describe the following...

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• Migration path for existing applications:
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Other information:

[the-spyke]

I have no idea why sourcemaps test fails on Windows in my branch. Probably, some dependency in the newer Babel returns broken Windows paths.

[JLHwung]

the AppVeyor CI error is not directly related to this change, but babel changed some path related behaviour from 7.2.0 to 7.8.0

[the-spyke]

@JLHwung C:/projects/babel-loader/test/fixtures/basic.js definitely looks wrong.

[nicolo-ribaudo]

Thanks!
We can investigate that failure separately if needed.

Approved too early 😅

[nicolo-ribaudo]

Can we differentiate this test based on process.version?

[the-spyke]

I was thinking about it, but I don't know if Node team will backport ESM to 12 or even 10 in the future and this test begin to fail. But okay, I'll redo it with semver check of Node version. As I remember 13.3 was the first one without --experimental flag.

[nicolo-ribaudo]

Let's only test in on 13.3 for now. If they'll backport it we can update the test.

[the-spyke]

Done

[the-spyke]

So, I've tried to overcome ESM loading on Window. But even in case of using file urls according to Node it fails. This time because of [email protected] even on Linux:

Module build failed (from ./lib/index.js):
   Error: Cannot find module 'file:///home/user/babel-loader/test/fixtures/babelrc.mjs' from '/home/user/babel-loader'
       at /home/user/babel-loader/node_modules/resolve/lib/async.js:97:35
       at processDirs (/home/user/babel-loader/node_modules/resolve/lib/async.js:244:39)
       at isdir (/home/user/babel-loader/node_modules/resolve/lib/async.js:251:32)
       at /home/user/babel-loader/node_modules/resolve/lib/async.js:23:69
       at FSReqCallback.oncomplete (fs.js:166:21)

[nicolo-ribaudo]

The windows bug should be fixed by babel/babel#11193.
I'm taking a look at the resolve failure.

[the-spyke]

@nicolo-ribaudo So, what do we do? Leave local path as it is now, rollback to pathToUrl and wait till it's fixed, or rollback and merge now?

[nicolo-ribaudo]

Let's wait for now, I'll release a patch version when there Babel PR is fixed.

[nishantjr]

Can this be merged now?

[nicolo-ribaudo]

Thanks!