Add test for implementation result types

Author: novemberborn

test-d/implementation-result.ts

@@ -0,0 +1,25 @@
+import test from '..';
+
+test('return a promise-like', t => {
+	return {
+		then(resolve) {
+			resolve?.();
+		}
+	};
+});
+
+test('return a subscribable', t => {
+	return {
+		subscribe({complete}) {
+			complete();
+		}
+	};
+});
+
+test('return anything else', t => {
+	return {
+		foo: 'bar',
+		subscribe() {},
+		then() {}
+	};
+});