Make sure UnknownXYExpressions are included when referenced as return…

… values in a MultiExpression (#3559)

src/ast/nodes/shared/MultiExpression.ts

@@ -1,9 +1,10 @@
 import { CallOptions } from '../../CallOptions';
 import { DeoptimizableEntity } from '../../DeoptimizableEntity';
-import { HasEffectsContext } from '../../ExecutionContext';
+import { HasEffectsContext, InclusionContext } from '../../ExecutionContext';
 import { ObjectPath, PathTracker } from '../../utils/PathTracker';
 import { LiteralValueOrUnknown, UnknownValue } from '../../values';
 import { ExpressionEntity } from './Expression';
+import { IncludeChildren } from './Node';
 
 export class MultiExpression implements ExpressionEntity {
 	included = false;
@@ -61,7 +62,15 @@ export class MultiExpression implements ExpressionEntity {
 		return false;
 	}
 
-	include(): void {}
+	include(context: InclusionContext, includeChildrenRecursively: IncludeChildren): void {
+		// This is only relevant to include values that do not have an AST representation,
+		// such as UnknownArrayExpression. Thus we only need to include them once.
+		for (const expression of this.expressions) {
+			if (!expression.included) {
+				expression.include(context, includeChildrenRecursively);
+			}
+		}
+	}
 
 	includeCallArguments(): void {}
 }

test/function/samples/chained-mutable-array-methods/_config.js

@@ -0,0 +1,9 @@
+const assert = require('assert');
+
+module.exports = {
+	description:
+		'recognizes side-effects when applying mutable array methods to chained array methods (#3555)',
+	exports(exports) {
+		assert.deepStrictEqual(exports.a, ['PASS']);
+	}
+};

test/function/samples/chained-mutable-array-methods/main.js

@@ -0,0 +1,5 @@
+function f(x) {
+	return (x ? [] : ['FAIL']).map(o => o);
+}
+export const a = f(0);
+a.splice(0, 1, 'PASS');