make import reassignment illegal @eventualbuddha

lib/6to5/transformation/modules/_default.js

@@ -9,7 +9,11 @@ function DefaultFormatter(file) {
   this.file = file;
 
   this.localExports = this.getLocalExports();
+  this.localImports = this.getLocalImports();
+
   this.remapAssignments();
+
+  this.checkImportAssignments();
 }
 
 DefaultFormatter.prototype.getLocalExports = function () {
@@ -27,6 +31,51 @@ DefaultFormatter.prototype.getLocalExports = function () {
   return localExports;
 };
 
+DefaultFormatter.prototype.getLocalImports = function () {
+  var localImports = {};
+
+  traverse(this.file.ast, {
+    enter: function (node) {
+      if (t.isImportDeclaration(node)) {
+        _.extend(localImports, t.getIds(node, true));
+      }
+    }
+  });
+
+  return localImports;
+};
+
+DefaultFormatter.prototype.checkImportAssignments = function () {
+  var localImports = this.localImports;
+  var file = this.file;
+
+  var isLocalReference = function (node, scope) {
+    return t.isIdentifier(node) && localImports[node.name];
+  };
+
+  var check = function (node) {
+    if (isLocalReference(node)) {
+      throw file.errorWithNode(node, "Illegal assignment of module import");
+    }
+  };
+
+  traverse(file.ast, {
+    enter: function (node, parent, scope) {
+      if (t.isAssignmentExpression(node)) {
+
+        var left = node.left;
+        if (t.isMemberExpression(left)) {
+          while (left.object) left = left.object;
+        }
+
+        check(left);
+      } else if (t.isDeclaration(node) && !t.isImportDeclaration(node)) {
+        _.each(t.getIds(node, true), check);
+      }
+    }
+  });
+};
+
 DefaultFormatter.prototype.remapExportAssignment = function (node) {
   return t.assignmentExpression(
     "=",

test/fixtures/transformation/es6-modules-common/disallow-import-remapping-2/actual.js

@@ -0,0 +1,2 @@
+import { foo } from "foo";
+var foo;

test/fixtures/transformation/es6-modules-common/disallow-import-remapping-2/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "Illegal assignment of module import"
+}

test/fixtures/transformation/es6-modules-common/disallow-import-remapping/actual.js

@@ -0,0 +1,2 @@
+import { foo } from "foo";
+foo = 1;

test/fixtures/transformation/es6-modules-common/disallow-import-remapping/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "Illegal assignment of module import"
+}