don't treat this.foo as possible namespace

src/ast/nodes/MemberExpression.js

@@ -33,7 +33,7 @@ export default class MemberExpression extends Node {
 		// TODO this code is a bit inefficient
 		const keypath = new Keypath( this );
 
-		if ( !keypath.computed ) {
+		if ( !keypath.computed && keypath.root.type === 'Identifier' ) {
 			let declaration = scope.findDeclaration( keypath.root.name );
 
 			while ( declaration.isNamespace && keypath.parts.length ) {

test/function/this-not-namespace/_config.js

@@ -0,0 +1,8 @@
+const assert = require( 'assert' );
+
+module.exports = {
+	description: 'does not treat this.foo as a possible namespace (#1258)',
+	exports: exports => {
+		assert.equal( typeof exports.Foo, 'function' );
+	}
+};

test/function/this-not-namespace/main.js

@@ -0,0 +1,5 @@
+export class Foo {
+	constructor ( name ) {
+		this.name = undefined;
+	}
+}