tsgo hangs with class/namespace merge + recursive types + declaration:true

[jasonkuhrt]

Summary

tsgo hangs indefinitely on code that tsc handles fine.

Reproduction

https://github.com/jasonkuhrt/repro-tsgo-hang

pnpm install
pnpm demo:works  # tsc completes
pnpm demo:hangs  # tsgo hangs

Workarounds

• Exporting the internal Thing_ type fixes hang
• Removing declaration:true from tsconfig fixes hang
• Removing namespace wrapper fixes hang

Minimal Code

tsconfig.json

{
  "compilerOptions": {
    "noEmit": true,
    "declaration": true
  },
  "include": ["repro.ts"]
}

repro.ts

type U<$A, $B, $C> =
  $B extends 0    ? any :
  $A extends any  ? Omit<$A, 0> & $C :
                    any

export class A {
  static fromString = <$input>(
    input: A.Thing<$input, A.NameEmpty>
  ) => null as any
}

export namespace A {
  export type Name = { b: any[]; a: any }
  export type NameEmpty = { b: []; a: null }

  type Add<$A extends Name, $B> =
    $A['a'] extends any
      ? U<$A, '', $B>
      : U<$A, '', $B>

  export type Thing<$A, $B extends Name> = Thing_<$A, $B>

  type Thing_<$A, $B extends Name> =
    $A extends `` ? $B :
    $A extends `${infer v} ${infer tail}` ? Thing_<tail, Add<$B, v>> :
    $A extends `${infer v}` ? Add<$B, v> :
    'unknown'
}

Environment

• tsgo: 7.0.0-dev.20251216.1 (@typescript/native-preview)
• tsc: 5.7.2

[so1ve]

Encountered this issue as well.

tsgo 7.0.0-dev.20251218.1 hangs while 7.0.0-dev.20251217.1 does not.

A workaround: pin tsgo to 7.0.0-dev.20251217.1 in your package.json and add "typescript.native-preview.tsdk": "./node_modules/@typescript/native-preview" to your settings.json.

[PaulRBerg]

I can reproduce a different "hang" mode in tsgo that happens before any TS program work, even for --help.

Repro (macOS 26.2 arm64, @typescript/native-preview 7.0.0-dev.20251218.1):

• In any directory without a go.mod (e.g. a workspace package dir like cli/), run tsgo --help or tsgo --noEmit.
• Process pegs a core (~100% CPU) and never exits.

SIGQUIT stack trace shows it stuck in go.mod discovery:

• github.com/microsoft/typescript-go/internal/repo.findGoMod (internal/repo/paths.go:30)
• repeatedly calling os.Stat("go.mod")

Workarounds:

• run from a directory that contains a go.mod (even a dummy one), or
• create a temp dir with go.mod and run tsgo --noEmit -p /abs/path/to/tsconfig.json from there.

Not sure if this is related to this type-system hang, but it makes tsgo unusable in typical JS monorepo package dirs unless you add go.mod everywhere.

If a maintainer confirms that this is a separate bug, I can open a separate issue with full logs + a minimal folder repro.

[jakebailey]

Are any of you able to git bisect?

[jakebailey]

If it's 18.1, the issue is #2411.

[jakebailey]

@PaulRBerg I think you cracked it; will copy your post to #2411

[JonasBa]

We are not using declaration: true, but I am also seeing tsgo hangs on Sentry repository.

[jakebailey]

If it's new in the latest nightly, then it's #2411 which will be fixed in 7.0.0-dev.20251218.3 (releasing as we speak).

[JonasBa]

Thanks @jakebailey, I can confirm that the new release fixes the issues. Appreciate the fast turnaround here, tsgo is awesome 🫶

[PaulRBerg]

can confirm that it works now, thx @jakebailey

tsgo rocks! ✌️