Bad parse for union type and function type

[Kingwl]

TypeScript Version: 3.4.0-dev.201xxxxx

Search Terms:
parser, arrow, union

Code

declare const a: string | (a: string, b: boolean) => string
declare const b: string | () => string
declare const c: string | () => string | number | () => boolean

Expected behavior:

declare const a: string | (a: string, b: boolean) => string
>a : string | ((a: string, b: boolean) => string)
>a : string
>b : boolean

declare const b: string | () => string
>b : string | (() => string)

// ?
declare const c: string | () => string | number | () => boolean
>c : string | (() => string | number | (() => boolean))

Actual behavior:
parse error

Playground Link:

Related Issues:
#25898

[dragomirtitian]

Are we sure not requiring the () around the function type is a good idea ? Better error messages as #25898 suggests is definitely a good idea, the current error is very unclear.

The problem is that looking at string | () => string | number | () => boolean it is not immediately obvious if the user means string | (() => string | number | (() => boolean)) or string | (() => string) | number | (() => boolean). I actually think that more users are probably trying to define the latter (not based on any data just gut feeling).

[RyanCavanaugh]

All on board for better error messages, but we really don't want to open the door further to confusing constructs like

declare const k: number | () => string | boolean;

[Kingwl]

But what about only for a better error？same as the previous issue