TypeScript: public/private class constructor argument does not transpile default parameter

[nstepien]

Bug Report

Current Behavior
The public/private keywords in a class constructor's argument prevents the default parameter from being transpiled.

Input Code

• REPL or Repo link if applicable: https://babeljs.io/repl/build/master#?babili=false&browsers=ie%2011&build=&builtIns=false&spec=false&loose=false&code_lz=KYDwDg9gTgLgBAE2AMwIYFcA28DGnUDOBcAoiKgLZibBwDeAUHHDhAHYExTo4zQAUYdACNMASxxxUUAOYBGOAF44bLJgCU9OAF8G2oA&debug=false&forceAllTransforms=false&shippedProposals=false&circleciRepo=&evaluate=false&fileSize=false&timeTravel=false&sourceType=module&lineWrap=true&presets=typescript%2Cenv&prettier=false&targets=&version=7.0.0&envVersion=7.0.0
  The typescript preset doesn't seem to work in the REPL, just add the @babel/plugin-transform-typescript to check the results.

export default class Example {
  constructor(public arg1 = null) { }
}

Expected behavior/code
I've enabled the env preset, configured to target ie11, so the default parameter should be transpiled to something ie11 can work with.

The transpiled code looks like this:

"use strict";

Object.defineProperty(exports, "__esModule", {
  value: true
});
exports.default = void 0;

function _classCallCheck(instance, Constructor) { if (!(instance instanceof Constructor)) { throw new TypeError("Cannot call a class as a function"); } }

var Example = function Example(arg1 = null) {
  _classCallCheck(this, Example);
};

exports.default = Example;

Babel Configuration (.babelrc, package.json, cli command)

{
  "presets": [
    ["@babel/env", {
      "useBuiltIns": "entry"
    }],
    "@babel/typescript",
    "@babel/react"
  ],
  "plugins": [
    ["@babel/transform-runtime", { "useESModules": true }],
    ["@babel/proposal-decorators", { "legacy": true }],
    ["@babel/proposal-class-properties", { "loose": true }]
  ]
}

Environment

• Babel version(s): v7.0.0
• Node/npm version: Node 10.10.0/npm 6.4.1
• OS: Windows 10
• Monorepo: yes
• How you are using Babel: loader

Possible Solution
Removing the public/private keywords does generate good code:

"use strict";

Object.defineProperty(exports, "__esModule", {
  value: true
});
exports.default = void 0;

function _classCallCheck(instance, Constructor) { if (!(instance instanceof Constructor)) { throw new TypeError("Cannot call a class as a function"); } }

var Example = function Example() {
  var arg1 = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : null;

  _classCallCheck(this, Example);
};

exports.default = Example;

You can also check how TS transpiles this piece of code: https://www.typescriptlang.org/play/#src=export%20default%20class%20Example%20%7B%0D%0A%20%20constructor(public%20arg1%20%3D%20null)%20%7B%20%7D%0D%0A%7D

define(["require", "exports"], function (require, exports) {
    "use strict";
    Object.defineProperty(exports, "__esModule", { value: true });
    var Example = /** @class */ (function () {
        function Example(arg1) {
            if (arg1 === void 0) { arg1 = null; }
            this.arg1 = arg1;
        }
        return Example;
    }());
    exports.default = Example;
});

Additional context/Screenshots
this.arg1 = arg1; is also missing from the constructor, as you can see from the TS transpilation result above. This is already addressed there: #7074 #8669 #8682

[babel-bot]

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