Fix problems with third-party Promise (#427)

Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>

lib/promise.js

@@ -1,8 +1,9 @@
 'use strict';
 
+const nativePromisePrototype = (async () => {})().constructor.prototype;
 const descriptors = ['then', 'catch', 'finally'].map(property => [
 	property,
-	Reflect.getOwnPropertyDescriptor(Promise.prototype, property)
+	Reflect.getOwnPropertyDescriptor(nativePromisePrototype, property)
 ]);
 
 // The return value is a mixin of `childProcess` and `Promise`

test/override-promise.js

@@ -0,0 +1,20 @@
+import path from 'path';
+import test from 'ava';
+
+process.env.PATH = path.join(__dirname, 'fixtures') + path.delimiter + process.env.PATH;
+
+// Can't use `test.before`, because `ava` needs `Promise`.
+// Can't use `import(…)` either, because `execa` is not an ES Module.
+const nativePromise = Promise;
+global.Promise = class BrokenPromise {
+	then() {
+		throw new Error('error');
+	}
+};
+const execa = require('..');
+global.Promise = nativePromise;
+
+test('should work with third-party Promise', async t => {
+	const {stdout} = await execa('noop', ['foo']);
+	t.is(stdout, 'foo');
+});