fix: babel 7.5, fixes #1292

[theKashey]

Related:

• Re-export statements are omitted from output using @babel/plugin-transform-typescript@7.5.0 babel/babel#10162
• Typescript plugin breaks in development mode gatsbyjs/gatsby#15423

I actually have to idea what this "fix" is fixing, but...

[codecov-io]

Codecov Report

Merging #1293 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1293   +/-   ##
=======================================
  Coverage   79.01%   79.01%           
=======================================
  Files          39       39           
  Lines        1625     1625           
  Branches      416      416           
=======================================
  Hits         1284     1284           
  Misses        277      277           
  Partials       64       64

Impacted Files	Coverage Δ
src/babel.dev.js	95.69% <100%> (ø)	⬆️

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[pieh]

I have absolutely no context on change, but will just share how this change impacts reproduction I use:

Here's input file: https://github.com/gatsbyjs/gatsby/blob/master/examples/using-typescript/src/layouts/index.tsx

And here's how applying change from this PR makes impact on output: https://www.diffchecker.com/wya2XyXe

And it fixes this particular test case at least.

Thanks for quick PR during the weekend!

[qiqiboy]

👍 Fix #1292

[nicolo-ribaudo]

(Babel team member here)

This PR is correct.

I'm sorry if v7.5 of @babel/plugin-transform-typescript broke compatibility with this plugin, but the only way of implementing many features in Babel is by assuming that the scope information is correct. I think that this missing registerDeclaration plugin never caused issues because the only transformation that ever happened to that const _default declaration was to transform it to var, and that plugin had been designed not to fail in case of missing bindings in the scope.

[nicolo-ribaudo]

I'm working on a fix on Babel side not to rely on scope information. I think that this PR should be merged anyway isnce it does the correct thing, but at least we won't break compatibility with older react-hot-loader versions.

[pieh]

Huge thanks @nicolo-ribaudo and @theKashey for fast turnaround on this ❤️