Typescript generics, TSX and trailing commas (prettier generates invalid syntax)

[jleclanche]

Background

I was trying to have a generic function in a tsx file, but was using const f = () ... syntax. When a single-argument generic is added to that style of function, the parser switches to TSX mode.
I came across this SO answer which explains that the parser will not switch to TSX mode if more than one generic argument is present.

So I discovered you can just add a trailing comma without having to have more than one argument. Problem is, prettier doesn't like that trailing comma and it gets cleaned up, which generates invalid tsx.

Reproduction

Run prettier over this:

type G<T> = any;
const myFunc = <T,>(arg1: G<T>) => false;

The generic's trailing comma gets removed, and it gets turned into the following invalid TSX (but valid TS):

type G<T> = any;
const myFunc = <T>(arg1: G<T>) => false;