assignments to properties of bindings is permitted

src/Statement.js

@@ -156,17 +156,30 @@ export default class Statement {
 
 	checkForWrites ( scope, node ) {
 		const addNode = ( node, disallowImportReassignments ) => {
+			let depth = 0; // determine whether we're illegally modifying a binding or namespace
+
 			while ( node.type === 'MemberExpression' ) {
 				node = node.object;
+				depth += 1;
 			}
 
 			// disallow assignments/updates to imported bindings and namespaces
-			if ( disallowImportReassignments && has( this.module.imports, node.name ) && !scope.contains( node.name ) ) {
+			if ( disallowImportReassignments ) {
+				const importSpecifier = this.module.imports[ node.name ];
+
+				if ( importSpecifier && !scope.contains( node.name ) ) {
+					const minDepth = importSpecifier.name === '*' ?
+						2 : // cannot do e.g. `namespace.foo = bar`
+						1;  // cannot do e.g. `foo = bar`, but `foo.bar = bar` is fine
+
+					if ( depth < minDepth ) {
 						const err = new Error( `Illegal reassignment to import '${node.name}'` );
 						err.file = this.module.path;
 						err.loc = getLocation( this.module.magicString.toString(), node.start );
 						throw err;
 					}
+				}
+			}
 
 			if ( node.type !== 'Identifier' ) {
 				return;

test/function/legal-import-modification/_config.js

@@ -0,0 +1,3 @@
+module.exports = {
+	description: 'assigning to properties of imported bindings is permitted'
+};

test/function/legal-import-modification/foo.js

@@ -0,0 +1 @@
+export default {};

test/function/legal-import-modification/main.js

@@ -0,0 +1,5 @@
+import foo from './foo';
+
+foo.modified = true;
+
+export default foo;