enhanced-resolve: `extensionAlias` is not applied to module exports

Let’s say we have a TypeScript mono repo with one package that depends on another. The actual TypeScript configuration is left out, because it’s not needed for this example.

webpack.config.js:

/** @type {import('webpack').Configuration} */
export default {
  entry: './packages/foo/index.ts',
  mode: 'development',
  resolve: {
    extensions: ['.js', '.ts'],
    extensionAlias: {
      '.js': ['.js', '.ts']
    }
  }
}

package.json:

{
  "type": "module",
  "workspaces": [
    "packages/*"
  ],
  "dependencies": {
    "webpack": "^5.0.0",
    "webpack-cli": "^4.0.0"
  }
}

foo/packages/package.json:

{
  "name": "foo",
  "type": "module",
  "exports": "./index.js"
}

foo/packages/index.ts:

import { bar } from 'bar'

console.log(bar)

bar/packages/package.json:

{
  "name": "bar",
  "type": "module",
  "exports": "./index.js"
}

bar/packages/index.ts:

export const bar = 'bar'

Now if we build using Webpack, it throws the following error:

~/Projects/bug$ webpack
asset main.js 2.34 KiB [emitted] (name: main)
runtime modules 274 bytes 1 module
./packages/foo/index.ts 44 bytes [built] [code generated]

ERROR in ./packages/foo/index.ts 1:0-25
Module not found: Error: Can't resolve 'bar' in '/home/remco/Projects/bug/packages/foo'
resolve 'bar' in '/home/remco/Projects/bug/packages/foo'
  Parsed request is a module
  using description file: /home/remco/Projects/bug/packages/foo/package.json (relative path: .)
    Field 'browser' doesn't contain a valid alias configuration
    resolve as module
      /home/remco/Projects/bug/packages/foo/node_modules doesn't exist or is not a directory
      /home/remco/Projects/bug/packages/node_modules doesn't exist or is not a directory
      looking for modules in /home/remco/Projects/bug/node_modules
        single file module
          using description file: /home/remco/Projects/bug/package.json (relative path: ./node_modules/bar)
            no extension
              Field 'browser' doesn't contain a valid alias configuration
              /home/remco/Projects/bug/node_modules/bar is not a file
            .js
              Field 'browser' doesn't contain a valid alias configuration
              /home/remco/Projects/bug/node_modules/bar.js doesn't exist
            .ts
              Field 'browser' doesn't contain a valid alias configuration
              /home/remco/Projects/bug/node_modules/bar.ts doesn't exist
        existing directory /home/remco/Projects/bug/node_modules/bar
          using description file: /home/remco/Projects/bug/node_modules/bar/package.json (relative path: .)
            using exports field: ./index.js
              using description file: /home/remco/Projects/bug/node_modules/bar/package.json (relative path: ./index.js)
                no extension
                  Field 'browser' doesn't contain a valid alias configuration
                  /home/remco/Projects/bug/node_modules/bar/index.js doesn't exist
                .js
                  Field 'browser' doesn't contain a valid alias configuration
                  /home/remco/Projects/bug/node_modules/bar/index.js.js doesn't exist
                .ts
                  Field 'browser' doesn't contain a valid alias configuration
                  /home/remco/Projects/bug/node_modules/bar/index.js.ts doesn't exist
                as directory
                  /home/remco/Projects/bug/node_modules/bar/index.js doesn't exist
      /home/remco/Projects/node_modules doesn't exist or is not a directory
      /home/remco/node_modules doesn't exist or is not a directory
      /home/node_modules doesn't exist or is not a directory
      /node_modules doesn't exist or is not a directory

This shows that extensionAlias isn’t applied when resolving import 'bar' from the foo project. This can be worked around by removing the exports field from package.json, but that will cause Node.js to show deprecation warnings if the package would be published and consumed.