The Bug

This repo demonstrates a minor bug with rollup-plugin-esbuild that sees it give a confusing error message in some circumstances when using async/await code.

An example of some code that will cause such a warning can be found in src/index.ts and is shown below:

export const myAsyncFunction = async (): Promise<void> => {
	await Promise.resolve('hello');
}

When bundling with Rollup and rollup-plugin-esbuild this will result in the following warning:

(!) Error when using sourcemap for reporting an error: Can't resolve original location of error.
src/index.ts (21:45)
(!) `this` has been rewritten to `undefined`
https://rollupjs.org/guide/en/#error-this-is-undefined
src/index.ts
19:   });
20: };
21: export const myAsyncFunction = () => __async(this, null, function* () {
                                                 ^
22:   yield Promise.resolve("hello");
23: });
created dist/main.cjs, dist/main.mjs in 39ms

This doesn't cause any issues in terms of the generated code's functionality but it is confusing.

To Reproduce

If you run yarn run build (after first running yarn install of course) in this repo you'll be presented with the warning shown above.

It doesn't seem to be an issue with esbuild itself since calling esbuild without rollup generates the following (run yarn run esbuildOnly to see for yourself):

// ... the __async helper function definition will be here but removed for clarity

// src/index.ts
var myAsyncFunction = () => __async(void 0, null, function* () {
  yield Promise.resolve("hello");
});
export {
  myAsyncFunction
};

As you can see above esbuild seems to output void 0 and not this. So the issue must be with rollup-plugin-esbuild.