Tracks side effects of thenables

src/ast/nodes/AwaitExpression.ts

@@ -1,3 +1,5 @@
+import { NormalizedTreeshakingOptions } from '../../rollup/types';
+import { NO_ARGS } from '../CallOptions';
 import { HasEffectsContext, InclusionContext } from '../ExecutionContext';
 import ArrowFunctionExpression from './ArrowFunctionExpression';
 import * as NodeType from './NodeType';
@@ -9,7 +11,20 @@ export default class AwaitExpression extends NodeBase {
 	type!: NodeType.tAwaitExpression;
 
 	hasEffects(context: HasEffectsContext): boolean {
-		return !context.ignore.returnAwaitYield || this.argument.hasEffects(context);
+		const { propertyReadSideEffects } = this.context.options
+			.treeshake as NormalizedTreeshakingOptions;
+		return (
+			!context.ignore.returnAwaitYield ||
+			this.argument.hasEffects(context) ||
+			this.argument.hasEffectsWhenCalledAtPath(
+				['then'],
+				{ args: NO_ARGS, thisParam: null, withNew: false },
+				context
+			) ||
+			(propertyReadSideEffects &&
+				(propertyReadSideEffects === 'always' ||
+					this.argument.hasEffectsWhenAccessedAtPath(['then'], context)))
+		);
 	}
 
 	include(context: InclusionContext, includeChildrenRecursively: IncludeChildren): void {

src/ast/nodes/shared/FunctionNode.ts

@@ -1,4 +1,4 @@
-import { CallOptions } from '../../CallOptions';
+import { CallOptions, NO_ARGS } from '../../CallOptions';
 import { BROKEN_FLOW_NONE, HasEffectsContext, InclusionContext } from '../../ExecutionContext';
 import { EVENT_CALLED, NodeEvent } from '../../NodeEvents';
 import FunctionScope from '../../scopes/FunctionScope';
@@ -56,7 +56,7 @@ export default class FunctionNode extends NodeBase {
 	}
 
 	getReturnExpressionWhenCalledAtPath(path: ObjectPath): ExpressionEntity {
-		return path.length === 0 ? this.scope.getReturnExpression() : UNKNOWN_EXPRESSION;
+		return !this.async && path.length === 0 ? this.scope.getReturnExpression() : UNKNOWN_EXPRESSION;
 	}
 
 	hasEffects(): boolean {
@@ -81,6 +81,18 @@ export default class FunctionNode extends NodeBase {
 		context: HasEffectsContext
 	): boolean {
 		if (path.length > 0) return true;
+		if (
+			this.async &&
+			this.scope
+				.getReturnExpression()
+				.hasEffectsWhenCalledAtPath(
+					['then'],
+					{ args: NO_ARGS, thisParam: null, withNew: false },
+					context
+				)
+		) {
+			return true;
+		}
 		for (const param of this.params) {
 			if (param.hasEffects(context)) return true;
 		}

src/ast/nodes/shared/knownGlobals.ts

@@ -191,10 +191,11 @@ const knownGlobals: GlobalDescription = {
 	Promise: {
 		__proto__: null,
 		[ValueProperties]: IMPURE,
-		all: PF,
+		all: O,
 		prototype: O,
-		race: PF,
-		resolve: PF
+		race: O,
+		reject: O,
+		resolve: O
 	},
 	propertyIsEnumerable: O,
 	Proxy: O,

test/form/samples/async-function-effects/_config.js

@@ -0,0 +1,5 @@
+const path = require('path');
+
+module.exports = {
+	description: 'tracks effects when awaiting thenables'
+};

test/form/samples/async-function-effects/_expected.js

@@ -0,0 +1,74 @@
+(async function () {
+	return {
+		then() {
+			console.log(1);
+		}
+	};
+})();
+
+(async function () {
+	await {
+		then: function () {
+			console.log(2);
+		}
+	};
+	return { then() {} };
+})();
+
+(async function () {
+	await {
+		get then() {
+			console.log(3);
+			return () => {};
+		}
+	};
+	return { then() {} };
+})();
+
+(async function () {
+	await {
+		get then() {
+			return () => console.log(4);
+		}
+	};
+	return { then() {} };
+})();
+
+(async function () {
+	await await {
+		then(resolve) {
+			resolve({
+				then() {
+					console.log(5);
+				}
+			});
+		}
+	};
+	return { then() {} };
+})();
+
+async function asyncIdentity(x) {
+	return x;
+}
+
+asyncIdentity({}); // no side effects - may be dropped
+
+const promise = asyncIdentity(6);
+
+promise.then(x => console.log(x));
+
+asyncIdentity({
+	then(success, fail) {
+		success(console.log(7));
+	}
+});
+
+asyncIdentity({
+	then(resolve) {
+		resolve({
+			then() {
+				console.log(8);
+			}
+		});
+	}
+});

test/form/samples/async-function-effects/main.js

@@ -0,0 +1,97 @@
+(async function () {
+	return {
+		then() {
+			console.log(1);
+		}
+	};
+})();
+
+// removed
+(async function () {
+	return { then() {} };
+})();
+
+(async function () {
+	await {
+		then: function () {
+			console.log(2);
+		}
+	};
+	return { then() {} };
+})();
+
+// removed
+(async function () {
+	await {
+		then: function () {}
+	};
+	return { then() {} };
+})();
+
+(async function () {
+	await {
+		get then() {
+			console.log(3);
+			return () => {};
+		}
+	};
+	return { then() {} };
+})();
+
+(async function () {
+	await {
+		get then() {
+			return () => console.log(4);
+		}
+	};
+	return { then() {} };
+})();
+
+// removed
+(async function () {
+	await {
+		get then() {
+			return () => {};
+		}
+	};
+	return { then() {} };
+})();
+
+(async function () {
+	await await {
+		then(resolve) {
+			resolve({
+				then() {
+					console.log(5);
+				}
+			});
+		}
+	};
+	return { then() {} };
+})();
+
+async function asyncIdentity(x) {
+	return x;
+}
+
+asyncIdentity({}); // no side effects - may be dropped
+
+const promise = asyncIdentity(6);
+
+promise.then(x => console.log(x));
+
+asyncIdentity({
+	then(success, fail) {
+		success(console.log(7));
+	}
+});
+
+asyncIdentity({
+	then(resolve) {
+		resolve({
+			then() {
+				console.log(8);
+			}
+		});
+	}
+});

test/form/samples/promises/_config.js

@@ -1,4 +1,3 @@
 module.exports = {
-	description: 'do not remove promise creations',
-	options: { output: { name: 'bundle' } }
+	description: 'do not remove promise creations'
 };

test/form/samples/promises/_expected.js

@@ -0,0 +1,49 @@
+Promise.resolve({
+	then() {
+		console.log(1);
+	}
+});
+
+Promise.resolve({
+	get then() {
+		return () => console.log(2);
+	}
+});
+
+Promise.reject('should be kept for uncaught rejections');
+
+Promise.all([
+	{
+		then() {
+			console.log(3);
+		}
+	},
+	null
+]);
+
+Promise.all([
+	null,
+	{
+		get then() {
+			return () => console.log(4);
+		}
+	}
+]);
+
+Promise.race([
+	{
+		then() {
+			console.log(5);
+		}
+	},
+	null
+]);
+
+Promise.race([
+	null,
+	{
+		get then() {
+			return () => console.log(6);
+		}
+	}
+]);