[fix] Improve `is_promise` handling (#8162)

* correctly handle promises that are of function type

* add License

Co-authored-by: Vilsol <me@vil.so>

src/runtime/internal/utils.ts

@@ -10,8 +10,10 @@ export function assign<T, S>(tar: T, src: S): T & S {
 	return tar as T & S;
 }
 
+// Adapted from https://github.com/then/is-promise/blob/master/index.js
+// Distributed under MIT License https://github.com/then/is-promise/blob/master/LICENSE
 export function is_promise<T = any>(value: any): value is PromiseLike<T> {
-	return value && typeof value === 'object' && typeof value.then === 'function';
+	return !!value && (typeof value === 'object' || typeof value === 'function') && typeof value.then === 'function';
 }
 
 export function add_location(element, file, line, column, char) {

test/runtime/samples/await-function-promise/_config.js

@@ -0,0 +1,19 @@
+const realPromise = Promise.resolve(42);
+
+const promise = () => {};
+promise.then = realPromise.then.bind(realPromise);
+promise.catch = realPromise.catch.bind(realPromise);
+
+export default {
+	props: {
+		promise
+	},
+
+	test({ assert, target }) {
+		return promise.then(() => {
+			assert.htmlEqual(target.innerHTML, `
+				<p>42</p>
+			`);
+		});
+	}
+};

test/runtime/samples/await-function-promise/main.svelte

@@ -0,0 +1,7 @@
+<script>
+	export let promise;
+</script>
+
+{#await promise then value}
+	<p>{JSON.stringify(value)}</p>
+{/await}