Use first binding for multiple var declarations (#5745)

* Use first binding for multiple var declarations
Since var declarations after initial binding have no effect, use the
first declaration. Fixes #2378

* Include hoisted function bindings

* Missing newline in expected.js

* Simplify constantViolations in new Binding on existing

* clarify comment language

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

@@ -13,6 +13,14 @@ 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;
+    }
+
     this.identifier = identifier;
     this.scope = scope;
     this.path = path;
@@ -26,14 +34,6 @@ export default class Binding {
     this.references = 0;
 
     this.clearValue();
-
-    if (existing) {
-      this.constantViolations = [].concat(
-        existing.path,
-        existing.constantViolations,
-        this.constantViolations,
-      );
-    }
   }
 
   constantViolations: Array<NodePath>;

packages/babel-traverse/test/fixtures/rename/parameter-redeclaration/actual.js

@@ -0,0 +1,11 @@
+function f(a, b) {
+  var a = "redeclared";
+  return b;
+}
+
+function g(a) {
+  function a() {
+    return "function, redeclared";
+  }
+  return a();
+}

packages/babel-traverse/test/fixtures/rename/parameter-redeclaration/expected.js

@@ -0,0 +1,12 @@
+function f(z, b) {
+  var z = "redeclared";
+  return b;
+}
+
+function g(z) {
+  function z() {
+    return "function, redeclared";
+  }
+
+  return z();
+}

packages/babel-traverse/test/fixtures/rename/parameter-redeclaration/options.json

@@ -0,0 +1,3 @@
+{
+  "plugins": ["./plugin"]
+}

packages/babel-traverse/test/fixtures/rename/parameter-redeclaration/plugin.js

@@ -0,0 +1,9 @@
+module.exports = function() {
+  return {
+    visitor: {
+      Function(path) {
+        path.scope.rename("a", "z");
+      }
+    }
+  };
+}