[flow] broken code emitted for type annotation with empty type parameters when the value is an arrow function and arrow-parens: 'avoid'

[bradzacher]

Prettier 2.7.1
Playground link

--parser flow
--arrow-parens avoid
--trailing-comma all

Input:

const xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx: xxxxxxxxxxxxxxxxxxxxxx<> =
  arg => null;

Output:

const xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx: xxxxxxxxxxxxxxxxxxxxxx<
  ,
> = arg => null;

Expected behavior:

Not-broken code. I'd suggest probably:

const xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx: xxxxxxxxxxxxxxxxxxxxxx<> =
  arg => null;

[bradzacher]

It's worth noting that the same code emits correctly for typescript and babel-ts, but not babel, babel-flow, or flow.

However the output for the TS parsers is pretty ugly - it includes an empty line!

const xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx: xxxxxxxxxxxxxxxxxxxxxx<

> = arg => null;