ts-loader forces 'isolatedModules' enabled when transpileOnly: true

[utc8]

Expected Behaviour

Successfully compile project with:

transpileOnly: true in ts-loader and "isolatedModules": false in tsconfig.json

Actual Behaviour

Steps to Reproduce the Problem

// src/index.ts
import { JumpType } from "./enum";
console.log("JumpType.AUTO", JumpType.AUTO);

// src/enum.ts
export const enum JumpType {
  INVALID = "INVALID",
  AUTO = "AUTO",
  CLICK = "CLICK",
}

// webpack.config.js
const path = require("path");
const webpack = require("webpack");

const rootDir = path.resolve(__dirname, ".");

module.exports = {
  entry: path.join(rootDir, "src/index.ts"),
  cache: false,
  mode: 'production',
  optimization: {
    minimize: false,
  },
  module: {
    rules: [
      {
        test: /\.tsx?$/,
        use: [
          {
            loader: "ts-loader",
            options: {
              transpileOnly: true,
            },
          },
        ],
      },
    ]
  },
  resolve: {
    extensions: ['.ts', '.tsx', '.js', '.json'],
  }
};

// tsconfig.json
{
  "compilerOptions": {
    "preserveConstEnums": false,
    "isolatedModules": false,
    "skipLibCheck": true,
    "skipDefaultLibCheck": true,
    "module": "esnext",
    "target": "es2015",
    "allowSyntheticDefaultImports": true,
    "baseUrl": "./",
  },
  "exclude": ["**/node_modules/"]
}

[elad-yosifon]

@utc8 AFAIK transpileOnly: true depends on isolatedModules: true since it aspires to be as fast as possible, thus forcing parallel compilation of source files.

If the files/modules are not isolated, you cannot parallelize the compilation safely (if there is no isolation, you will need to "see" all files at once, resolve some edge cases [like const enums...], and then compile - which is not truly parallel).