Fix scope of function body when var redeclares param

packages/babel-helper-call-delegate/src/index.js

@@ -18,7 +18,11 @@ const visitor = {
   },
 };
 
-export default function(path: NodePath, scope = path.scope) {
+export default function(
+  path: NodePath,
+  scope = path.scope,
+  shouldHoistVariables = true,
+) {
   const { node } = path;
   const container = t.functionExpression(
     null,
@@ -31,8 +35,9 @@ export default function(path: NodePath, scope = path.scope) {
   let callee = container;
   let args = [];
 
-  // todo: only hoist if necessary
-  hoistVariables(path, id => scope.push({ id }));
+  if (shouldHoistVariables) {
+    hoistVariables(path, id => scope.push({ id }));
+  }
 
   const state = {
     foundThis: false,

packages/babel-plugin-transform-parameters/src/params.js

@@ -64,6 +64,31 @@ export default function convertFunctionParams(path, loose) {
   for (let i = 0; i < params.length; i++) {
     const param = params[i];
 
+    for (const name of Object.keys(param.getBindingIdentifiers())) {
+      const constantViolations = scope.bindings[name]?.constantViolations;
+      if (constantViolations) {
+        for (const redeclarator of constantViolations) {
+          const node = redeclarator.node;
+          // If a constant violation is a var or a function declaration,
+          // we first check to see if it's a var without an init.
+          // If so, we remove that declarator.
+          // Otherwise, we have to wrap it in an IIFE.
+          switch (node.type) {
+            case "VariableDeclarator":
+              if (node.init === null) {
+                redeclarator.remove();
+              } else {
+                state.iife = true;
+              }
+              break;
+            case "FunctionDeclaration":
+              state.iife = true;
+              break;
+          }
+        }
+      }
+    }
+
     const paramIsAssignmentPattern = param.isAssignmentPattern();
     if (paramIsAssignmentPattern && (loose || node.kind === "set")) {
       const left = param.get("left");
@@ -146,7 +171,8 @@ export default function convertFunctionParams(path, loose) {
   path.ensureBlock();
 
   if (state.iife) {
-    body.push(callDelegate(path, scope));
+    // we don't want to hoist the inner declarations up
+    body.push(callDelegate(path, scope, false));
     path.set("body", t.blockStatement(body));
   } else {
     path.get("body").unshiftContainer("body", body);

packages/babel-plugin-transform-parameters/test/fixtures/parameters/fn-decl-same-as-param/input.js

@@ -0,0 +1,5 @@
+function foo(a = 2) {
+  function a() {
+
+  }
+}

packages/babel-plugin-transform-parameters/test/fixtures/parameters/fn-decl-same-as-param/output.js

@@ -0,0 +1,6 @@
+function foo() {
+  var a = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : 2;
+  return function () {
+    function a() {}
+  }();
+}

packages/babel-plugin-transform-parameters/test/fixtures/parameters/var-no-init-same-as-param/input.js

@@ -0,0 +1,6 @@
+function f(a = 2, b = 3) {
+  var a, b = 4;
+  var a;
+  var b;
+  return a + b;
+}

packages/babel-plugin-transform-parameters/test/fixtures/parameters/var-no-init-same-as-param/output.js

@@ -0,0 +1,8 @@
+function f() {
+  var a = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : 2;
+  var b = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : 3;
+  return function () {
+    var b = 4;
+    return a + b;
+  }();
+}

packages/babel-plugin-transform-parameters/test/fixtures/parameters/var-no-init-then-var-with-init-same-as-param/input.js

@@ -0,0 +1,5 @@
+function f(a = 2, b = 3) {
+  var a;
+  var { a } = { a: 4 };
+  return a + b;
+}

packages/babel-plugin-transform-parameters/test/fixtures/parameters/var-no-init-then-var-with-init-same-as-param/output.js

@@ -0,0 +1,11 @@
+function f() {
+  var a = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : 2;
+  var b = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : 3;
+  return function () {
+    var _a = {
+      a: 4
+    },
+        a = _a.a;
+    return a + b;
+  }();
+}

packages/babel-plugin-transform-parameters/test/fixtures/parameters/var-same-as-array-pattern-param/input.js

@@ -0,0 +1,5 @@
+function foo({a, b}) {
+  var a = 3;
+  var c = 2;
+  var d = a + b + c;
+}

packages/babel-plugin-transform-parameters/test/fixtures/parameters/var-same-as-array-pattern-param/output.js

@@ -0,0 +1,9 @@
+function foo(_ref) {
+  var a = _ref.a,
+      b = _ref.b;
+  return function () {
+    var a = 3;
+    var c = 2;
+    var d = a + b + c;
+  }();
+}

packages/babel-plugin-transform-parameters/test/fixtures/parameters/var-same-as-object-pattern-param/input.js

@@ -0,0 +1,5 @@
+function foo([a, b]) {
+  var a = 3;
+  var c = 2;
+  var d = a + b + c;
+}

packages/babel-plugin-transform-parameters/test/fixtures/parameters/var-same-as-object-pattern-param/output.js

@@ -0,0 +1,11 @@
+function foo(_ref) {
+  var _ref2 = babelHelpers.slicedToArray(_ref, 2),
+      a = _ref2[0],
+      b = _ref2[1];
+
+  return function () {
+    var a = 3;
+    var c = 2;
+    var d = a + b + c;
+  }();
+}

packages/babel-plugin-transform-parameters/test/fixtures/parameters/var-same-as-param-closure/exec.js

@@ -0,0 +1,8 @@
+var x = 1
+function foo(x, y = function () { x = 2 }) {
+  var x = 3
+  y()
+  expect(x).toBe(3);
+}
+foo()
+expect(x).toBe(1);

packages/babel-plugin-transform-parameters/test/fixtures/parameters/var-same-as-param/input.js

@@ -0,0 +1,3 @@
+function foo(a = 2) {
+  var a = 1;
+}

packages/babel-plugin-transform-parameters/test/fixtures/parameters/var-same-as-param/output.js

@@ -0,0 +1,6 @@
+function foo() {
+  var a = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : 2;
+  return function () {
+    var a = 1;
+  }();
+}

packages/babel-plugin-transform-parameters/test/fixtures/parameters/var-with-init-then-var-no-init-same-as-param/input.js

@@ -0,0 +1,6 @@
+function f(a = 2, b = 3) {
+  var { a } = { a: 4 };
+  var a;
+  var b;
+  return a + b;
+}

packages/babel-plugin-transform-parameters/test/fixtures/parameters/var-with-init-then-var-no-init-same-as-param/output.js

@@ -0,0 +1,11 @@
+function f() {
+  var a = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : 2;
+  var b = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : 3;
+  return function () {
+    var _a = {
+      a: 4
+    },
+        a = _a.a;
+    return a + b;
+  }();
+}