TypeScript syntax error in combination with arrow functions, generics and async/await

Is this a bug report?

Yes, this is a bug report.

I get a syntax error with the following code, which I reduced as much as possible to just trigger the error:

const first = async <X extends {}>() => {
  await something();
};

const second = async <Y extends {}>() => {};

Did you try recovering your dependencies?

Yes, but recovering didn’t help.

Output of yarn --version:

1.13.0

Environment

$ npx create-react-app --info

Environment Info:

  System:
    OS: macOS 10.14.2
    CPU: x64 Intel(R) Core(TM) i5-7360U CPU @ 2.30GHz
  Binaries:
    Node: 11.9.0 - /usr/local/bin/node
    Yarn: 1.13.0 - /usr/local/bin/yarn
    npm: 6.7.0 - /usr/local/bin/npm
  Browsers:
    Chrome: 71.0.3578.98
    Firefox: 64.0.2
    Safari: 12.0.2
  npmPackages:
    react: ^16.8.1 => 16.8.1 
    react-dom: ^16.8.1 => 16.8.1 
    react-scripts: 2.1.3 => 2.1.3 
  npmGlobalPackages:
    create-react-app: Not Found

Steps to Reproduce

See “Reproducible Demo”.

Expected Behavior

I wouldn’t expect a syntax error (other errors with this code, yes, but no syntax error)

AFAIK the code is valid TypeScript.

Actual Behavior

Syntax error:

Failed to compile.

./src/index.tsx
SyntaxError: /Users/sebu/Desktop/2019-02-08-cra-ts-syntaxerror/src/index.tsx: Unexpected token, expected ";" (5:24)

  3 | };
  4 | 
> 5 | const second = async <Y extends {}>() => {};
    |                         ^
  6 | 

As colorized screenshot:

Reproducible Demo

Reproducible repo at https://github.com/sbusch/2019-02-08-cra-ts-syntaxerror which I stripped down as much as possible.

Problem is in src/index.tsx:

Steps to reproduce:

git clone https://github.com/sbusch/2019-02-08-cra-ts-syntaxerror.git

cd 2019-02-08-cra-ts-syntaxerror

yarn install

BROWSER=none yarn start

[ianschmitz]

You can try our latest 3.0 alpha which has updated versions of babel: https://github.com/facebook/create-react-app/issues/6475

[sbusch]

I can confirm his is no longer an issue with CRA 3.0 (I’m currently using 3.0.1).