From 4b85b05839017ab2abc03ba2d1e875a63f002890 Mon Sep 17 00:00:00 2001
From: Sebastian McKenzie <sebmck@gmail.com>
Date: Fri, 5 Jun 2015 14:08:18 +0100
Subject: [PATCH] use actual parameter reference for non-last default
 parameters - fixes #1690

---
 .../helpers/get-function-arity.js             |  5 +-
 .../templates/default-parameter-assign.js     |  1 +
 .../transformers/es6/parameters.default.js    | 60 +++++++++++++------
 .../before-last/actual.js                     |  3 +
 .../before-last/expected.js                   |  5 ++
 5 files changed, 54 insertions(+), 20 deletions(-)
 create mode 100644 src/babel/transformation/templates/default-parameter-assign.js
 create mode 100644 test/core/fixtures/transformation/es6.parameters.default/before-last/actual.js
 create mode 100644 test/core/fixtures/transformation/es6.parameters.default/before-last/expected.js

diff --git a/src/babel/transformation/helpers/get-function-arity.js b/src/babel/transformation/helpers/get-function-arity.js
index 98a4e42839..e59d86aad4 100644
--- a/src/babel/transformation/helpers/get-function-arity.js
+++ b/src/babel/transformation/helpers/get-function-arity.js
@@ -3,7 +3,10 @@ import * as t from "../../types";
 export default function (node) {
   var lastNonDefault = 0;
   for (var i = 0; i < node.params.length; i++) {
-    if (!t.isAssignmentPattern(node.params[i])) lastNonDefault = i + 1;
+    var param = node.params[i];
+    if (!t.isAssignmentPattern(param) && !t.isRestElement(param)) {
+      lastNonDefault = i + 1;
+    }
   }
   return lastNonDefault;
 }
diff --git a/src/babel/transformation/templates/default-parameter-assign.js b/src/babel/transformation/templates/default-parameter-assign.js
new file mode 100644
index 0000000000..847a500e42
--- /dev/null
+++ b/src/babel/transformation/templates/default-parameter-assign.js
@@ -0,0 +1 @@
+if (VARIABLE_NAME === undefined) VARIABLE_NAME = DEFAULT_VALUE;
diff --git a/src/babel/transformation/transformers/es6/parameters.default.js b/src/babel/transformation/transformers/es6/parameters.default.js
index 547c91b754..04f6f7391c 100644
--- a/src/babel/transformation/transformers/es6/parameters.default.js
+++ b/src/babel/transformation/transformers/es6/parameters.default.js
@@ -1,4 +1,5 @@
 import callDelegate from "../../helpers/call-delegate";
+import getFunctionArity from "../../helpers/get-function-arity";
 import * as util from  "../../../util";
 import * as t from "../../../types";
 
@@ -24,37 +25,54 @@ var iifeVisitor = {
 export function Func/*tion*/(node, parent, scope, file) {
   if (!hasDefaults(node)) return;
 
+  // ensure it's a block, useful for arrow functions
   t.ensureBlock(node);
 
+  var state = {
+    iife: false,
+    scope: scope
+  };
+
   var body = [];
 
+  //
   var argsIdentifier = t.identifier("arguments");
   argsIdentifier._shadowedFunctionLiteral = true;
 
-  var lastNonDefaultParam = 0;
-
-  var state = { iife: false, scope: scope };
-
-  var pushDefNode = function (left, right, i) {
-    var defNode = util.template("default-parameter", {
-      VARIABLE_NAME: left,
-      DEFAULT_VALUE: right,
-      ARGUMENT_KEY:  t.literal(i),
-      ARGUMENTS:     argsIdentifier
-    }, true);
+  // push a default parameter definition
+  function pushDefNode(left, right, i) {
+    var defNode;
+    if (exceedsLastNonDefault(i) || t.isPattern(left) || file.transformers["es6.spec.blockScoping"].canTransform()) {
+      defNode = util.template("default-parameter", {
+        VARIABLE_NAME: left,
+        DEFAULT_VALUE: right,
+        ARGUMENT_KEY:  t.literal(i),
+        ARGUMENTS:     argsIdentifier
+      }, true);
+    } else {
+      defNode = util.template("default-parameter-assign", {
+        VARIABLE_NAME: left,
+        DEFAULT_VALUE: right
+      }, true);
+    }
     defNode._blockHoist = node.params.length - i;
     body.push(defNode);
-  };
+  }
 
+  // check if an index exceeds the functions arity
+  function exceedsLastNonDefault(i) {
+    return i + 1 > lastNonDefaultParam;
+  }
+
+  //
+  var lastNonDefaultParam = getFunctionArity(node);
+
+  //
   var params = this.get("params");
   for (var i = 0; i < params.length; i++) {
     var param = params[i];
 
     if (!param.isAssignmentPattern()) {
-      if (!param.isRestElement()) {
-        lastNonDefaultParam = i + 1;
-      }
-
       if (!param.isIdentifier()) {
         param.traverse(iifeVisitor, state);
       }
@@ -69,9 +87,13 @@ export function Func/*tion*/(node, parent, scope, file) {
     var left  = param.get("left");
     var right = param.get("right");
 
-    var placeholder = scope.generateUidIdentifier("x");
-    placeholder._isDefaultPlaceholder = true;
-    node.params[i] = placeholder;
+    if (exceedsLastNonDefault(i) || left.isPattern()) {
+      var placeholder = scope.generateUidIdentifier("x");
+      placeholder._isDefaultPlaceholder = true;
+      node.params[i] = placeholder;
+    } else {
+      node.params[i] = left.node;
+    }
 
     if (!state.iife) {
       if (right.isIdentifier() && scope.hasOwnBinding(right.node.name)) {
diff --git a/test/core/fixtures/transformation/es6.parameters.default/before-last/actual.js b/test/core/fixtures/transformation/es6.parameters.default/before-last/actual.js
new file mode 100644
index 0000000000..d5f34c085b
--- /dev/null
+++ b/test/core/fixtures/transformation/es6.parameters.default/before-last/actual.js
@@ -0,0 +1,3 @@
+function foo(a = "foo", b) {
+
+}
diff --git a/test/core/fixtures/transformation/es6.parameters.default/before-last/expected.js b/test/core/fixtures/transformation/es6.parameters.default/before-last/expected.js
new file mode 100644
index 0000000000..cc754fd872
--- /dev/null
+++ b/test/core/fixtures/transformation/es6.parameters.default/before-last/expected.js
@@ -0,0 +1,5 @@
+"use strict";
+
+function foo(a, b) {
+  if (a === undefined) a = "foo";
+}