Redeclaring a variable counts as a modification (#6219)

* Redeclaring a variable counts as a modification.

Fixes #6217.

* Remove "existing" logic from Binding.

Was added in #5745, but no longer triggered since 6536e605a.

packages/babel-plugin-transform-es2015-parameters/test/fixtures/parameters/rest-member-expression-deoptimisation/expected.js

@@ -141,9 +141,11 @@ function arrayDestructure() {
     rest[_key15] = arguments[_key15];
   }
 
-  var _x = babelHelpers.slicedToArray(x, 1);
+  var _x = x;
 
-  rest[0] = _x[0];
+  var _x2 = babelHelpers.slicedToArray(_x, 1);
+
+  rest[0] = _x2[0];
 }
 
 function forOf() {

packages/babel-traverse/src/scope/binding.js

@@ -12,15 +12,7 @@ import type NodePath from "../path";
  */
 
 export default class Binding {
-  constructor({ existing, identifier, scope, path, kind }) {
-    // this if condition is true only if the re-binding does not result in an error
-    // e.g. if rebinding kind is 'var' or 'hoisted', and previous was 'param'
-    if (existing) {
-      // we maintain the original binding but update constantViolations
-      existing.constantViolations = existing.constantViolations.concat(path);
-      return existing;
-    }
-
+  constructor({ identifier, scope, path, kind }) {
     this.identifier = identifier;
     this.scope = scope;
     this.path = path;

packages/babel-traverse/src/scope/index.js

@@ -535,13 +535,17 @@ export default class Scope {
 
         parent.references[name] = true;
 
-        this.bindings[name] = new Binding({
-          identifier: id,
-          existing: local,
-          scope: this,
-          path: bindingPath,
-          kind: kind,
-        });
+        // A redeclaration of an existing variable is a modification
+        if (local) {
+          this.registerConstantViolation(bindingPath);
+        } else {
+          this.bindings[name] = new Binding({
+            identifier: id,
+            scope: this,
+            path: bindingPath,
+            kind: kind,
+          });
+        }
       }
     }
   }

packages/babel-traverse/test/scope.js

@@ -73,6 +73,20 @@ describe("scope", function() {
       );
     });
 
+    it("variable constantness", function() {
+      assert.ok(getPath("var a = 1;").scope.getBinding("a").constant === true);
+      assert.ok(
+        getPath("var a = 1; a = 2;").scope.getBinding("a").constant === false,
+      );
+      assert.ok(
+        getPath("var a = 1, a = 2;").scope.getBinding("a").constant === false,
+      );
+      assert.ok(
+        getPath("var a = 1; var a = 2;").scope.getBinding("a").constant ===
+          false,
+      );
+    });
+
     it("purity", function() {
       assert.ok(
         getPath("({ x: 1 })")