Compiler hangs

[falsandtru]

But looks like CPU and Memory usage is not increasing.

TypeScript Version: master

Search Terms:

Code

export type Prepend<Elm, T extends unknown[]> =
  T extends unknown ?
  ((arg: Elm, ...rest: T) => void) extends ((...args: infer T2) => void) ? T2 :
  never :
  never;
export type ExactExtract<T, U> = T extends U ? U extends T ? T : never : never;

type Conv<T, U = T> =
  { 0: [T]; 1: Prepend<T, Conv<ExactExtract<U, T>>>;}[U extends T ? 0 : 1];

Expected behavior:

pass

Actual behavior:

hang

Playground Link:

Related Issues:

[sandersn]

Also fails on 3.5.1

[weswigham]

The type expands so well:

I'm actually surprised we just hang rather than quickly OOM. Apparently a Conv<ExactExtract<U, T>> is not a simple type, and determining if it's assignable to an unknown[] is not so easy. :P

[fatcerberus]

@weswigham Wait, why does that type expand recursively like that? Shouldn't that have been caught by the instantiation depth limiter?

[weswigham]

Oh, it is. The depth limit is 50 instances of the same type and there's like 10 different types, so that's like 500 types deep that it can easily go.