diff --git a/src/ast/nodes/BlockStatement.js b/src/ast/nodes/BlockStatement.js
index 1aad2de6573..c344510567c 100644
--- a/src/ast/nodes/BlockStatement.js
+++ b/src/ast/nodes/BlockStatement.js
@@ -1,12 +1,12 @@
 import Statement from './shared/Statement.js';
 import BlockScope from '../scopes/BlockScope';
-import UndefinedIdentifier from './shared/UndefinedIdentifier';
+import { UNKNOWN_ASSIGNMENT } from '../values';
 
 export default class BlockStatement extends Statement {
 	bindImplicitReturnExpressionToScope () {
 		const lastStatement = this.body[ this.body.length - 1 ];
 		if ( !lastStatement || lastStatement.type !== 'ReturnStatement' ) {
-			this.scope.addReturnExpression( new UndefinedIdentifier() );
+			this.scope.addReturnExpression( UNKNOWN_ASSIGNMENT );
 		}
 	}
 
diff --git a/src/ast/nodes/UnaryExpression.js b/src/ast/nodes/UnaryExpression.js
index 4e50e2b875b..0b79f177472 100644
--- a/src/ast/nodes/UnaryExpression.js
+++ b/src/ast/nodes/UnaryExpression.js
@@ -1,6 +1,5 @@
 import Node from '../Node.js';
-import { UNKNOWN_VALUE } from '../values';
-import UndefinedIdentifier from './shared/UndefinedIdentifier';
+import { UNKNOWN_ASSIGNMENT, UNKNOWN_VALUE } from '../values';
 import ExecutionPathOptions from '../ExecutionPathOptions';
 
 const operators = {
@@ -16,7 +15,7 @@ const operators = {
 export default class UnaryExpression extends Node {
 	bindNode () {
 		if ( this.operator === 'delete' ) {
-			this.argument.bindAssignmentAtPath( [], new UndefinedIdentifier(), ExecutionPathOptions.create() );
+			this.argument.bindAssignmentAtPath( [], UNKNOWN_ASSIGNMENT, ExecutionPathOptions.create() );
 		}
 	}
 
diff --git a/src/ast/nodes/shared/FunctionNode.js b/src/ast/nodes/shared/FunctionNode.js
index b3116c07d8a..3a7d1a16470 100644
--- a/src/ast/nodes/shared/FunctionNode.js
+++ b/src/ast/nodes/shared/FunctionNode.js
@@ -51,6 +51,11 @@ export default class FunctionNode extends Node {
 			|| this.body.hasEffects( innerOptions );
 	}
 
+	includeInBundle () {
+		this.scope.variables.arguments.includeVariable();
+		return super.includeInBundle();
+	}
+
 	initialiseNode () {
 		this.prototypeObject = new VirtualObjectExpression();
 	}
diff --git a/src/ast/nodes/shared/UndefinedIdentifier.js b/src/ast/nodes/shared/UndefinedIdentifier.js
deleted file mode 100644
index 46a561a09f6..00000000000
--- a/src/ast/nodes/shared/UndefinedIdentifier.js
+++ /dev/null
@@ -1,19 +0,0 @@
-import Node from '../../Node';
-
-export default class UndefinedIdentifier extends Node {
-	hasEffects () {
-		return false;
-	}
-
-	hasEffectsWhenAccessedAtPath ( path ) {
-		return path.length > 0;
-	}
-
-	hasEffectsWhenAssignedAtPath ( path ) {
-		return path.length > 0;
-	}
-
-	toString () {
-		return 'undefined';
-	}
-}
diff --git a/src/ast/scopes/ModuleScope.js b/src/ast/scopes/ModuleScope.js
index ebdc5c58cfd..6d9c472e997 100644
--- a/src/ast/scopes/ModuleScope.js
+++ b/src/ast/scopes/ModuleScope.js
@@ -2,7 +2,7 @@ import { forOwn } from '../../utils/object.js';
 import relativeId from '../../utils/relativeId.js';
 import Scope from './Scope.js';
 import LocalVariable from '../variables/LocalVariable';
-import UndefinedIdentifier from '../nodes/shared/UndefinedIdentifier';
+import { UNKNOWN_ASSIGNMENT } from '../values';
 
 export default class ModuleScope extends Scope {
 	constructor ( module ) {
@@ -12,7 +12,7 @@ export default class ModuleScope extends Scope {
 		} );
 
 		this.module = module;
-		this.variables.this = new LocalVariable( 'this', null, new UndefinedIdentifier() );
+		this.variables.this = new LocalVariable( 'this', null, UNKNOWN_ASSIGNMENT );
 	}
 
 	deshadow ( names ) {
diff --git a/src/ast/scopes/Scope.js b/src/ast/scopes/Scope.js
index 28e9a409fcf..cc8548e6b67 100644
--- a/src/ast/scopes/Scope.js
+++ b/src/ast/scopes/Scope.js
@@ -1,7 +1,7 @@
 import { blank, keys } from '../../utils/object.js';
 import LocalVariable from '../variables/LocalVariable';
 import ExportDefaultVariable from '../variables/ExportDefaultVariable';
-import UndefinedIdentifier from '../nodes/shared/UndefinedIdentifier';
+import { UNKNOWN_ASSIGNMENT } from '../values';
 
 export default class Scope {
 	constructor ( options = {} ) {
@@ -28,7 +28,7 @@ export default class Scope {
 			variable.addDeclaration( identifier );
 			options.init && variable.bindAssignmentAtPath( [], options.init );
 		} else {
-			this.variables[ name ] = new LocalVariable( identifier.name, identifier, options.init || new UndefinedIdentifier() );
+			this.variables[ name ] = new LocalVariable( identifier.name, identifier, options.init || UNKNOWN_ASSIGNMENT );
 		}
 		return this.variables[ name ];
 	}
diff --git a/src/ast/values.js b/src/ast/values.js
index 04ce1d31a7d..8d97af1d64f 100644
--- a/src/ast/values.js
+++ b/src/ast/values.js
@@ -4,8 +4,9 @@ export const UNKNOWN_ASSIGNMENT = {
 	type: 'UNKNOWN',
 	bindAssignmentAtPath: () => {},
 	bindCallAtPath: () => {},
+	forEachReturnExpressionWhenCalledAtPath: () => {},
 	hasEffectsWhenAccessedAtPath: path => path.length > 0,
-	hasEffectsWhenAssignedAtPath: () => true,
+	hasEffectsWhenAssignedAtPath: path => path.length > 0,
 	hasEffectsWhenCalledAtPath: () => true,
 	someReturnExpressionWhenCalledAtPath: () => true,
 	toString: () => '[[UNKNOWN]]'
diff --git a/src/ast/variables/ArgumentsVariable.js b/src/ast/variables/ArgumentsVariable.js
index 4199623a79d..0dd1e0c7b68 100644
--- a/src/ast/variables/ArgumentsVariable.js
+++ b/src/ast/variables/ArgumentsVariable.js
@@ -26,11 +26,10 @@ export default class ArgumentsVariable extends LocalVariable {
 	}
 
 	hasEffectsWhenAssignedAtPath ( path, options ) {
-		if ( path.length === 0 ) {
-			return true;
-		}
-		return getParameterVariable( path, options )
-			.hasEffectsWhenAssignedAtPath( path.slice( 1 ), options );
+		return path.length === 0
+			|| this.included
+			|| getParameterVariable( path, options )
+				.hasEffectsWhenAssignedAtPath( path.slice( 1 ), options );
 	}
 
 	hasEffectsWhenCalledAtPath ( path, callOptions, options ) {
diff --git a/src/ast/variables/ReplaceableInitStructuredAssignmentTracker.js b/src/ast/variables/ReplaceableInitStructuredAssignmentTracker.js
index 96d28f0189e..83ed45bb8ba 100644
--- a/src/ast/variables/ReplaceableInitStructuredAssignmentTracker.js
+++ b/src/ast/variables/ReplaceableInitStructuredAssignmentTracker.js
@@ -1,5 +1,6 @@
 import StructuredAssignmentTracker from './StructuredAssignmentTracker';
 import LocalVariable from './LocalVariable';
+import ExecutionPathOptions from '../ExecutionPathOptions';
 
 export default class ReplaceableInitStructuredAssignmentTracker extends StructuredAssignmentTracker {
 	constructor () {
@@ -9,13 +10,13 @@ export default class ReplaceableInitStructuredAssignmentTracker extends Structur
 
 	addAtPath ( path, assignment ) {
 		if ( path.length > 0 ) {
-			this._init.bindAssignmentAtPath( path, assignment );
+			this._init.bindAssignmentAtPath( path, assignment, ExecutionPathOptions.create() );
 		}
 		super.addAtPath( path, assignment );
 	}
 
 	addInit ( assignment ) {
-		this._init.bindAssignmentAtPath( [], assignment );
+		this._init.bindAssignmentAtPath( [], assignment, ExecutionPathOptions.create() );
 	}
 
 	forEachAtPath ( path, callback ) {
diff --git a/src/ast/variables/StructuredAssignmentTracker.js b/src/ast/variables/StructuredAssignmentTracker.js
index d1b2500460f..d591dc9865e 100644
--- a/src/ast/variables/StructuredAssignmentTracker.js
+++ b/src/ast/variables/StructuredAssignmentTracker.js
@@ -69,18 +69,6 @@ export default class StructuredAssignmentTracker {
 		}
 	}
 
-	hasEqualAtPath ( path, equalAssignment ) {
-		if ( path.length === 0 ) {
-			return Array.from( this._assignments.get( SET_KEY ) ).find( assignment => equalAssignment.equals( assignment ) );
-		} else {
-			const [ nextPath, ...remainingPath ] = path;
-			if ( !this._assignments.has( nextPath ) ) {
-				return false;
-			}
-			return this._assignments.get( nextPath ).hasAtPath( remainingPath, equalAssignment );
-		}
-	}
-
 	someAtPath ( path, predicateFunction ) {
 		const [ nextPath, ...remainingPath ] = path;
 		return Array.from( this._assignments.get( SET_KEY ) ).some( assignment => predicateFunction( path, assignment ) )