Fix error in rest parameter length optimization (#3573)

If there aren’t enough arguments to get to the offset index, we would
return an negative length.

packages/babel-plugin-transform-es2015-parameters/src/rest.js

@@ -13,10 +13,14 @@ let buildRest = template(`
   }
 `);
 
-let loadRest = template(`
+let restIndex = template(`
   ARGUMENTS.length <= INDEX ? undefined : ARGUMENTS[INDEX]
 `);
 
+let restLength = template(`
+  ARGUMENTS.length <= OFFSET ? 0 : ARGUMENTS.length - OFFSET
+`);
+
 let memberExpressionOptimisationVisitor = {
   Scope(path, state) {
     // check if this scope has a local binding that will shadow the rest parameter
@@ -155,21 +159,18 @@ function optimiseIndexGetter(path, argsId, offset) {
     index = t.binaryExpression("+", path.parent.property, t.numericLiteral(offset));
   }
 
-  path.parentPath.replaceWith(loadRest({
+  path.parentPath.replaceWith(restIndex({
     ARGUMENTS: argsId,
     INDEX: index,
   }));
 }
 
-function optimiseLengthGetter(path, argsLengthExpression, argsId, offset) {
+function optimiseLengthGetter(path, argsId, offset) {
   if (offset) {
-    path.parentPath.replaceWith(
-      t.binaryExpression(
-        "-",
-        argsLengthExpression,
-        t.numericLiteral(offset),
-      )
-    );
+    path.parentPath.replaceWith(restLength({
+      ARGUMENTS: argsId,
+      OFFSET: t.numericLiteral(offset),
+    }));
   } else {
     path.replaceWith(argsId);
   }
@@ -183,10 +184,6 @@ export let visitor = {
     let rest = node.params.pop().argument;
 
     let argsId = t.identifier("arguments");
-    let argsLengthExpression = t.memberExpression(
-      argsId,
-      t.identifier("length"),
-    );
 
     // otherwise `arguments` will be remapped in arrow functions
     argsId._shadowedFunctionLiteral = path;
@@ -230,7 +227,7 @@ export let visitor = {
             optimiseIndexGetter(path, argsId, state.offset);
             break;
           case "lengthGetter":
-            optimiseLengthGetter(path, argsLengthExpression, argsId, state.offset);
+            optimiseLengthGetter(path, argsId, state.offset);
             break;
           default:
             path.replaceWith(argsId);

packages/babel-plugin-transform-es2015-parameters/test/fixtures/parameters/rest-length/exec.js

@@ -0,0 +1,8 @@
+var length = function (a, b, ...items) {
+  return items.length;
+};
+
+assert.equal(length(), 0);
+assert.equal(length(1), 0);
+assert.equal(length(1, 2), 0);
+assert.equal(length(1, 2, 3), 1);

packages/babel-plugin-transform-es2015-parameters/test/fixtures/parameters/rest-length/expected.js

@@ -1,6 +1,6 @@
 var t = function (f) {
   arguments.length <= 1 ? undefined : arguments[1];
-  arguments.length <= arguments.length - 1 - 1 + 1 ? undefined : arguments[arguments.length - 1 - 1 + 1];
+  arguments.length <= (arguments.length <= 1 ? 0 : arguments.length - 1) - 1 + 1 ? undefined : arguments[(arguments.length <= 1 ? 0 : arguments.length - 1) - 1 + 1];
 };
 
 function t(f) {
@@ -11,4 +11,4 @@ function t(f) {
   items;
   items[0];
   items[items.length - 1];
-}
+}

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

@@ -63,11 +63,11 @@ var b = function () {
 };
 
 var b = function (foo) {
-  return (arguments.length - 1) * 2;
+  return (arguments.length <= 1 ? 0 : arguments.length - 1) * 2;
 };
 
 var b = function (foo, baz) {
-  return arguments.length - 2;
+  return arguments.length <= 2 ? 0 : arguments.length - 2;
 };
 
 function x() {
@@ -193,4 +193,4 @@ function postfixDecrement() {
   }
 
   rest[0]--;
-}
+}