include called functions that mutate their inputs

src/Declaration.js

@@ -2,7 +2,7 @@ import { blank, keys } from './utils/object.js';
 import testForSideEffects from './utils/testForSideEffects.js';
 
 export default class Declaration {
-	constructor ( node ) {
+	constructor ( node, isParam ) {
 		if ( node ) {
 			if ( node.type === 'FunctionDeclaration' ) {
 				this.isFunctionDeclaration = true;
@@ -15,6 +15,7 @@ export default class Declaration {
 
 		this.statement = null;
 		this.name = null;
+		this.isParam = isParam;
 
 		this.isReassigned = false;
 		this.aliases = [];
@@ -32,6 +33,7 @@ export default class Declaration {
 	}
 
 	testForSideEffects ( strongDependencies ) {
+		// TODO memoize this
 		if ( !this.statement || !this.functionNode ) return;
 		return testForSideEffects( this.functionNode.body, this.functionNode._scope, this.statement, strongDependencies );
 	}

src/ast/Scope.js

@@ -46,7 +46,7 @@ export default class Scope {
 		if ( options.params ) {
 			options.params.forEach( param => {
 				extractNames( param ).forEach( name => {
-					this.declarations[ name ] = new Declaration();
+					this.declarations[ name ] = new Declaration( param, true );
 				});
 			});
 		}

src/utils/testForSideEffects.js

@@ -8,9 +8,7 @@ let pureFunctions = {};
 [
 	// TODO add others to this list from https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects
 	'Array', 'Array.isArray',
-
 	'Error', 'EvalError', 'InternalError', 'RangeError', 'ReferenceError', 'SyntaxError', 'TypeError', 'URIError',
-
 	'Object', 'Object.keys'
 ].forEach( name => pureFunctions[ name ] = true );
 
@@ -51,8 +49,6 @@ export default function testForSideEffects ( node, scope, statement, strongDepen
 					} else if ( !pureFunctions[ node.callee.name ] ) {
 						hasSideEffect = true;
 					}
-
-					// TODO does function mutate inputs that are needed?
 				}
 
 				else if ( node.callee.type === 'MemberExpression' ) {
@@ -83,8 +79,12 @@ export default function testForSideEffects ( node, scope, statement, strongDepen
 				let subject = node[ modifierNodes[ node.type ] ];
 				while ( subject.type === 'MemberExpression' ) subject = subject.object;
 
-				if ( !scope.findDeclaration( subject.name ) ) {
-					const declaration = statement.module.trace( subject.name );
+				let declaration = scope.findDeclaration( subject.name );
+
+				if ( declaration ) {
+					hasSideEffect = declaration.isParam;
+				} else {
+					declaration = statement.module.trace( subject.name );
 
 					if ( !declaration || declaration.isExternal || declaration.isUsed ) {
 						hasSideEffect = true;