Deoptimize return values of async functions

src/ast/nodes/ArrowFunctionExpression.ts

@@ -20,6 +20,7 @@ export default class ArrowFunctionExpression extends NodeBase {
 	preventChildBlockScope!: true;
 	scope!: ReturnValueScope;
 	type!: NodeType.tArrowFunctionExpression;
+	private deoptimizedReturn = false;
 
 	createScope(parentScope: Scope): void {
 		this.scope = new ReturnValueScope(parentScope, this.context);
@@ -37,7 +38,18 @@ export default class ArrowFunctionExpression extends NodeBase {
 	deoptimizeThisOnEventAtPath(): void {}
 
 	getReturnExpressionWhenCalledAtPath(path: ObjectPath): ExpressionEntity {
-		return !this.async && path.length === 0 ? this.scope.getReturnExpression() : UNKNOWN_EXPRESSION;
+		if (path.length !== 0) {
+			return UNKNOWN_EXPRESSION;
+		}
+		if (this.async) {
+			if (!this.deoptimizedReturn) {
+				this.deoptimizedReturn = true;
+				this.scope.getReturnExpression().deoptimizePath(UNKNOWN_PATH);
+				this.context.requestTreeshakingPass();
+			}
+			return UNKNOWN_EXPRESSION;
+		}
+		return this.scope.getReturnExpression();
 	}
 
 	hasEffects(): boolean {

src/ast/nodes/AwaitExpression.ts

@@ -34,5 +34,6 @@ export default class AwaitExpression extends NodeBase {
 	protected applyDeoptimizations(): void {
 		this.deoptimized = true;
 		this.argument.deoptimizePath(UNKNOWN_PATH);
+		this.context.requestTreeshakingPass();
 	}
 }

src/ast/nodes/shared/FunctionNode.ts

@@ -21,6 +21,7 @@ export default class FunctionNode extends NodeBase {
 	params!: PatternNode[];
 	preventChildBlockScope!: true;
 	scope!: FunctionScope;
+	private deoptimizedReturn = false;
 	private isPrototypeDeoptimized = false;
 
 	createScope(parentScope: FunctionScope): void {
@@ -57,7 +58,18 @@ export default class FunctionNode extends NodeBase {
 	}
 
 	getReturnExpressionWhenCalledAtPath(path: ObjectPath): ExpressionEntity {
-		return !this.async && path.length === 0 ? this.scope.getReturnExpression() : UNKNOWN_EXPRESSION;
+		if (path.length !== 0) {
+			return UNKNOWN_EXPRESSION;
+		}
+		if (this.async) {
+			if (!this.deoptimizedReturn) {
+				this.deoptimizedReturn = true;
+				this.scope.getReturnExpression().deoptimizePath(UNKNOWN_PATH);
+				this.context.requestTreeshakingPass();
+			}
+			return UNKNOWN_EXPRESSION;
+		}
+		return this.scope.getReturnExpression();
 	}
 
 	hasEffects(): boolean {

test/function/samples/track-async-arrow-function-return-value/_config.js

@@ -0,0 +1,10 @@
+const assert = require('assert');
+
+module.exports = {
+	description: 'tracks object mutations of async arrow function return values',
+	async exports(exports) {
+		assert.strictEqual(exports.toggled, false);
+		await exports.test();
+		assert.strictEqual(exports.toggled, true);
+	}
+};

test/function/samples/track-async-arrow-function-return-value/main.js

@@ -0,0 +1,18 @@
+const fn = async () => {
+	const obj = {
+		test() {
+			if (typeof obj.testfn === 'function') {
+				obj.testfn();
+			}
+		}
+	};
+	return obj;
+};
+
+export let toggled = false;
+
+export const test = async function () {
+	const obj = await fn();
+	obj.testfn = () => (toggled = true);
+	obj.test();
+};

test/function/samples/track-async-function-return-value/_config.js

@@ -0,0 +1,10 @@
+const assert = require('assert');
+
+module.exports = {
+	description: 'tracks object mutations of async function return values',
+	async exports(exports) {
+		assert.strictEqual(exports.toggled, false);
+		await exports.test();
+		assert.strictEqual(exports.toggled, true);
+	}
+};

test/function/samples/track-async-function-return-value/main.js

@@ -0,0 +1,18 @@
+async function fn() {
+	const obj = {
+		test() {
+			if (typeof obj.testfn === 'function') {
+				obj.testfn();
+			}
+		}
+	};
+	return obj;
+}
+
+export let toggled = false;
+
+export const test = async function () {
+	const obj = await fn();
+	obj.testfn = () => (toggled = true);
+	obj.test();
+};

test/function/samples/track-awaited-object/main.js

@@ -6,7 +6,6 @@ function fn() {
 			}
 		}
 	};
-
 	return obj;
 }