moonc linker ICE with nested JsPromise[JsPromise[T]]

[bikallem]

Summary

The moonc linker crashes with Moonc.Basic_hashf.Make(Key).Key_not_found(_) when code produces a nested JsPromise[JsPromise[JsValue]] type. This happens when a .then() callback returns a JsPromise instead of a plain value.

Minimal reproduction

// moon.pkg
import { "bikallem/webapi" }
options("is-main": true, "supported-targets": ["js"])

// main.mbt
fn main {
  @webapi.window
  .fetch("http://example.com")
  .then(fn(response : @webapi.Response) {
    response.text().then(fn(body : String) -> @webapi.JsValue {
      println(body)
      @webapi.JsValue::undefined()
    })
  })
  |> ignore
}

moon check --target js passes, but moon build --target js crashes the linker:

Error: Moonc.Basic_hashf.Make(Key).Key_not_found(_)
moonc version: v0.8.2+8cca5f22a

Root cause

JsPromise::then[T, U] returns JsPromise[U]. When the callback returns JsPromise[JsValue], the result type is JsPromise[JsPromise[JsValue]] — a nested #external generic. The linker's hash table fails to look up this nested type key.

Workaround

Ignore the inner promise so the outer callback returns Unit:

fn main {
  @webapi.window
  .fetch("http://example.com")
  .then(fn(response : @webapi.Response) {
    response.text().then(fn(body : String) {
      println(body)
    })
    |> ignore  // outer callback returns Unit, not JsPromise
  })
  |> ignore
}

This builds successfully. Behavior is identical since JavaScript flattens nested promises at runtime.

Notes

• Reproducible with moonc v0.8.2+8cca5f22a, moon 0.1.20260209
• Not specific to fetch — any code producing JsPromise[JsPromise[T]] should trigger it
• This is a moonc compiler bug, filed here for visibility since webapi's JsPromise::then signature makes it easy to hit