Improve the type assertion for is.asyncFunction

package.json

@@ -59,7 +59,7 @@
 		"rxjs": "^6.4.0",
 		"tempy": "^0.3.0",
 		"ts-node": "^8.3.0",
-		"typescript": "^3.4.1",
+		"typescript": "^3.7.2",
 		"xo": "^0.25.3",
 		"zen-observable": "^0.8.8"
 	},

source/index.ts

@@ -152,8 +152,7 @@ is.promise = (value: unknown): value is Promise<unknown> => is.nativePromise(val
 
 is.generatorFunction = isObjectOfType<GeneratorFunction>(TypeName.GeneratorFunction);
 
-// eslint-disable-next-line @typescript-eslint/ban-types
-is.asyncFunction = isObjectOfType<Function>(TypeName.AsyncFunction);
+is.asyncFunction = (value: unknown): value is ((...args: any[]) => Promise<unknown>) => getObjectType(value) === TypeName.AsyncFunction;
 
 // eslint-disable-next-line no-prototype-builtins, @typescript-eslint/ban-types
 is.boundFunction = (value: unknown): value is Function => is.function_(value) && !value.hasOwnProperty('prototype');

test/test.ts

@@ -560,6 +560,12 @@ test('is.promise', t => {
 
 test('is.asyncFunction', t => {
 	testType(t, 'asyncFunction', ['function']);
+
+	const fixture = async () => {};
+	if (is.asyncFunction(fixture)) {
+		// eslint-disable-next-line promise/prefer-await-to-then
+		t.true(is.function_(fixture().then));
+	}
 });
 
 test('is.generator', t => {