TypeScript + JSX (TSX): required trailing commas are stripped from type parameters

[OliverJAsh]

Prettier 1.18.2
Playground link

Input:

const fn = <T,>(t: T) => t;

Output:
This is invalid syntax for a .tsx file. TypeScript needs the comma because without it, the syntax is ambiguous.

const fn = <T>(t: T) => t;

• Generic usage reported as JSX Error microsoft/TypeScript#15713 (comment)
• https://stackoverflow.com/questions/32308370/what-is-the-syntax-for-typescript-arrow-functions-with-generics#comment99104831_45576880

Expected behavior:

const fn = <T,>(t: T) => t;

[sosukesuzuki]

this issue is duplicated to #6114 . Also, fixed in #6115 and #6190 .
But, it behavior depends on file extension(.tsx), so we cannot see it in Playground (Playground is not supported file extension).

If you run prettier in CLI and file extension is tsx, syntax error does not happen.

[OliverJAsh]

Thanks!