Ensure parameter defaults are deoptimized

src/ast/nodes/ArrowFunctionExpression.ts

@@ -25,6 +25,7 @@ export default class ArrowFunctionExpression extends FunctionBase {
 	}
 
 	hasEffects(): boolean {
+		if (!this.deoptimized) this.applyDeoptimizations();
 		return false;
 	}
 

src/ast/nodes/shared/FunctionBase.ts

@@ -175,6 +175,7 @@ export default abstract class FunctionBase extends NodeBase implements Deoptimiz
 		includeChildrenRecursively: IncludeChildren,
 		{ includeWithoutParameterDefaults }: InclusionOptions = BLANK
 	): void {
+		if (!this.deoptimized) this.applyDeoptimizations();
 		this.included = true;
 		const { brokenFlow } = context;
 		context.brokenFlow = BROKEN_FLOW_NONE;
@@ -244,6 +245,16 @@ export default abstract class FunctionBase extends NodeBase implements Deoptimiz
 		super.parseNode(esTreeNode);
 	}
 
+	protected applyDeoptimizations() {
+		// We currently do not track deoptimizations of default values, deoptimize them
+		// just as we deoptimize call arguments
+		for (const param of this.params) {
+			if (param instanceof AssignmentPattern) {
+				param.right.deoptimizePath(UNKNOWN_PATH);
+			}
+		}
+	}
+
 	protected abstract getObjectEntity(): ObjectEntity;
 }
 

src/ast/nodes/shared/FunctionNode.ts

@@ -39,6 +39,7 @@ export default class FunctionNode extends FunctionBase {
 	}
 
 	hasEffects(): boolean {
+		if (!this.deoptimized) this.applyDeoptimizations();
 		return !!this.id?.hasEffects();
 	}
 

test/function/samples/parameter-defaults/function-as-default/_config.js

@@ -0,0 +1,3 @@
+module.exports = {
+	description: 'deoptimizes functions supplied as default values'
+};

test/function/samples/parameter-defaults/function-as-default/main.js

@@ -0,0 +1,4 @@
+const foo = (a = 'fallback') => a;
+const bar = (a = foo) => a;
+
+assert.strictEqual(bar()(), 'fallback');