Typescript: Flow analysis fails when using try catch block in IIFEs

[SanderRonde]

TypeScript Version: 3.8.0-beta onwards

Search Terms: return, iife, error, flow anaylsis, try catch

Code

const x = (() => {
  try { return 'x'; } catch (e) {
    return null;
  }
})();

// any code here is unreachable according to TS

Expected behavior:
Typescript marks code as reachable

Actual behavior:
Typescript wrongfully assumes that any code after the block is unreachable

Playground Link:
https://www.typescriptlang.org/play/?ts=3.8-Beta&ssl=11&ssc=14&pln=17&pc=4#code/DYUwLgBAngXBBGB7RoCGA7A3AWAFB4DMBXdAYzAEtF0IBnRAW3AAsL0BzAFWdTAGFERYABMAYqgrAAFAEo4tMACc27CAG88ECBQIQpUGes1aIYZosQB3COhDWAooouKpAchBPEi1zJy4tAL7GiuBEijSuCsocrn5B+LjEZJTUEATIsvJKKkb+EKTUChAAHhAAvHqy5QB8uSamilB19RAhYGE09ExmKty8AkJiEtK+xoH5vKTMeiCGGnn1bR02QsB+9fGBMrJ+xjp6pWVHK8DAhkvhEK4EqKfwqKQA1rF4waGXxXFAA

Related Issues:
None found

Some more info:
I initially found this issue in version 3.9.0-dev.2000215 but found it to also be present in 3.8.0-beta. It it not present in the current stable version (3.7.5).

[ahejlsberg]

@RyanCavanaugh We should consider pulling this into 3.8.

[uhyo]

This bug is preventing our team from upgrading to TS 3.8.
Hope the fix could be backported to 3.8! 😃

[hirofumii]

If not my mistake, this bug does not seem to be fixed in 3.8.3 :(

@ahejlsberg I'm glad if you can confirm.
thanks.

Playground Link:
https://www.typescriptlang.org/play/#code/DYUwLgBAngXBBGB7RoCGA7A3AWAFB4DMBXdAYzAEtF0IBnRAW3AAsL0BzAFWdTAGFERYABMAYqgrAAFAEo4tMACc27CAG88ECBQIQpUGes1aIYZosQB3COhDWAooouKpAchBPEi1zJy4tAL7GiuBEijSuCsocrn5B+LjEZJTUEATIsvJKKkb+EKTUChAAHhAAvHqy5QB8uSamilB19RAhYGE09ExmKty8AkJiEtK+xoH5vKTMeiCGGnn1bR02QsB+9fGBMrJ+xjp6pWVHK8DAhkvhEK4EqKfwqKQA1rF4waGXxXFAA

[bradzacher]

It looks like this has been fixed in the nightly at least (the playground doesn't have a 3.9.0-beta version option) - 3.9.0-dev.20200328

https://www.typescriptlang.org/play/?ts=3.9.0-dev.20200328&ssl=1&ssc=1&pln=12&pc=1#code/GYVwdgxgLglg9mABMMAVAFgQygWRgc3SgwCc4B3AIRCgBE4BTAZzCgAoBKRAb0QF8AUBARMoiAHIB5WgFEA+gEEAykpkAlVHKWo1ASQDCmmQEUAqgoAycyQAV1C1JLWIAvIjadXAPh4DE-xCgSAE9fAPDkNCxcAiJSCmo6RhZ2DgBuPwC+RAhsCHR3BhISLm5MiMQSBigQEiQikgA6OAAHIuw4EgzwwXCodDJyRDAGIZlizrYAInAGAA826AYAE0QmEAgIZiZQABtETCYmItgEKfSBPg5ODKEROF2GRt24fDYAcnQYd4ugA

[ahejlsberg]

The fix didn't make it into 3.8.3. It is fixed in 3.9 beta.