4.0.1 causes problems with Bluebird

[tprobinson]

I've gotten an error from Bluebird when using execa at version 4.0.1:

TypeError: this._then is not a function
    at Promise.then (~/.config/yarn/global/node_modules/bluebird/js/release/promise.js:154:17)
    at ChildProcess.then (~/.config/yarn/global/node_modules/execa/lib/promise.js:13:25)

This works fine if I downgrade execa to 4.0.0.

[ehmicky]

Thanks for reporting this problem @tprobinson!

@fisker This appears to be related to #403, do you know what the problem might be?

[fisker]

I don't know, but I'll take a look.

[fisker]

I found the reason, here we use Promise.prototype, which is bluebird.protype(global.Promise is override) in this case, and here we use async function returns a native Promise, so we are calling bluebird#then on native Promise.

Old version use Promise#then directly, so there is no problem.

I see possible solutions

1. use old logic, change it back to

(...args) => promise()[property](...args);

2. use this to get the nativePromise

const nativePromise = (async () => {})().constructor;

3. wrap async function to return a global.Promise(bluebird in this case) instance, I think this is bad idea.

Which one do you prefer?

[ehmicky]

I think 1. might be simpler. 2. is a little more involved, but it works too.

[fisker]

I think 2 better, some old Promise lib may don't have .finally, or broken like I did in the test.