Parse error on arrow function with type parameters

[mightyiam]

What version of TypeScript are you using?

2.5.3

What version of typescript-eslint-parser are you using?

8.0.0

What code were you trying to parse?

<T>() => {
  return null
}

What did you expect to happen?

Happy happy parse parse. I notice we have this fixture but... error.

What happened?

/home/shahar/src/pick-path/src/parsing-error.ts
  2:2  error  Parsing error: Expression expected

✖ 1 problem (1 error, 0 warnings)

[JamesHenry]

Hmm interesting. It seems like this was a genuine parse error in TypeScript 2.4.x (i.e. nothing to do with this parser)

http://astexplorer.net/#/gist/69f89d5c7455a7713684a53d49bdf7b1/51082ff9da1db4f52d729a17d1cf8b70b52e9ae7

...but the TypeScript playground, which is using 2.5.x, does not complain.

It is odd slightly code in isolation I must say 😄 Do you have a more complete example of the context in which this was used?

[JamesHenry]

I also cannot seem to reproduce this error on master of this project, using TypeScript 2.5.3. When I set up a spec containing the source code you have provided, it seems to be able to produce an AST and does not throw.

Please can you see if you can reproduce?

[mightyiam]

Here is my actual use:
https://github.com/mightyiam/pick-path/pull/16.

[JamesHenry]

Ah, I see, using it as the value of a declaration makes sense 😄 I figured you couldn't have been using it in isolation like that.

Thanks for following up, I will investigate this further

[JamesHenry]

This must be something to do with your full setup, because your source code does not produce a parse error in the latest version of this parser using TypeScript 2.5.3.

As you pointed out, we do already have a fixture which covers this as well. I will see if I can see anything obvious in your setup as a whole.

[JamesHenry]

It seems like this will need to be investigated in your project https://github.com/mightyiam/eslint-config-standard-with-typescript? Have you tried narrowing down the issue in there and tweaking the config to get the minimal possible repro?

It might well be one of the custom rules from eslint-plugin-typescript is the root cause of this...

[mightyiam]

I'll do a minimal reproduction PR in this project.

[mightyiam]

The following code is all that it takes to trigger the error:

<T>(): T => {}

The error in this case is:

file: 'file:///home/shahar/src/pick-path/src/index.ts'
severity: 'Error'
message: 'Parsing error: JSX element 'T' has no corresponding closing tag.'
at: '1,1'
source: 'eslint'

When I remove from .eslintrc.json the property:

  "extends": "eslint-config-standard-with-typescript"

There is no error. Also, extending from eslint:recommended, also gives no error. Any clues here?

[mightyiam]

A JSX parser... Where? How?

[mightyiam]

This seems to avoid the error:

  "parserOptions": {
    "ecmaFeatures": {
      "jsx": false
    }
  }

[j-f1]

It appears to be interpreting the code like this:

<T> // JSX opening element
(): T => // raw JSX text
{} // an empty JSX expression container
// [!] missing </T> closing element

Given that this error is thrown by the TypeScript parser, it appears to be an issue with TypeScript itself, not just this plugin.