Parse error on generic arrow function

[WhiteAbeLincoln]

What version of TypeScript are you using? ^2.8.3

What version of typescript-eslint-parser are you using? ^15.0.0

What code were you trying to parse?

export const somes = <T>(options: ReadonlyArray<Option<T>>): ReadonlyArray<T> =>
  options.filter(isSome).map(o => o.value)

What did you expect to happen? There not to be a parse error

What happened? Eslint reported Parsing error: Identifier expected on Option<T>:

export const somes = <T>(options: ReadonlyArray<Option<T>>): ReadonlyArray<T> =>
                                                ^^^^^^^^^
  options.filter(isSome).map(o => o.value)

Changing this to a function statement instead of a arrow function removes the parse error. These are both valid:

export const somes = function<T>(options: ReadonlyArray<Option<T>>): ReadonlyArray<T> {
  return options.filter(isSome).map(o => o.value)
}

export function somes<T>(options: ReadonlyArray<Option<T>>): ReadonlyArray<T> {
  return options.filter(isSome).map(o => o.value)
}

Additionally, removing the type from the parameter changes the error to JSX element 'T' has no corresponding closing tag on the return type ReadonlyArray<T>:

export const somes = <T>(options: Option<T>[]): ReadonlyArray<T> =>
                                                ^^^^^^^^^^^^^^ 
  options.filter(isSome).map(o => o.value)

This is not a .tsx file, so the parser should probably not be attempting to parse types as JSX tags

[gmathieu]

I noticed the same error with the following:

import { getMiddleware } from 'lib/env'
import { createStore as reduxCreateStore, applyMiddleware, StoreCreator, Reducer } from 'redux'
import thunk from 'redux-thunk'

const storeCreator: StoreCreator = <S>(reducer: Reducer<S>) => {
    return reduxCreateStore(reducer, applyMiddleware(thunk, ...getMiddleware()))
}

export default storeCreator

lib/create-store.ts
  6:4  error  Parsing error: Expression expected

[JamesHenry]

Does your configuration have jsx: true?

If so it is highly likely: https://github.com/eslint/typescript-eslint-parser/issues/517

You can reproduce the same behaviour with the TypeScript parser directly on https://astexplorer.net/

[JamesHenry]

Hopefully this was covered by the resolution to #517, feel free to reopen if not!