Description

When a wasm-gc FFI stub function takes a closure (function type) as a parameter, the compiler rejects closures whose internal signature contains String, Array[T], or enum types with error [4042] Invalid stub type.

Closures passed to FFI stubs should be treated as opaque externref values on wasm-gc, regardless of their internal type signatures. The closure's parameter/return types are a MoonBit-level concern and should not affect the wasm import signature — the closure itself is just an externref.

Closures that work: those containing only #external types (externref) or wasm value types (Int, Double, UInt, etc.)

Closures that fail: those containing String, Array[T], or enum types as parameters or return types

Minimal Reproduction

moon.mod.json:

{
  "name": "blem/closure-stub-bug",
  "version": "0.1.0",
  "moonc-opt": "v0.8.1"
}

moon.pkg.json:

{
  "is-main": true,
  "supported-targets": ["wasm-gc"]
}

main.mbt:

#external
pub type Event

#external
pub type EventListener

// --- These work (externref or wasm value types inside closure) ---

fn ok_externref(callback : (Event) -> Unit) -> EventListener = "m" "a"
fn ok_no_args(callback : () -> Unit) -> EventListener = "m" "b"
fn ok_int(callback : (Int) -> Unit) -> EventListener = "m" "c"
fn ok_double(callback : (Double) -> Unit) -> EventListener = "m" "d"
fn ok_ret_externref(callback : (Event) -> Event) -> EventListener = "m" "e"

// --- These fail with [4042] "Invalid stub type" ---

fn fail_string_param(callback : (String) -> Unit) -> EventListener = "m" "f"
fn fail_string_return(callback : (Event) -> String) -> EventListener = "m" "g"
fn fail_array_param(callback : (Array[Event]) -> Unit) -> EventListener = "m" "h"

fn main {
  ignore(ok_externref)
  ignore(ok_no_args)
  ignore(ok_int)
  ignore(ok_double)
  ignore(ok_ret_externref)
  ignore(fail_string_param)
  ignore(fail_string_return)
  ignore(fail_array_param)
}

Actual behavior

$ moon build --target wasm-gc

Error: [4042]
 fn fail_string_param(callback : (String) -> Unit) -> EventListener = "m" "f"
                                 ────────┬───────
                                         ╰───────── Invalid stub type.

Error: [4042]
 fn fail_string_return(callback : (Event) -> String) -> EventListener = "m" "g"
                                  ────────┬────────
                                          ╰────────── Invalid stub type.

Error: [4042]
 fn fail_array_param(callback : (Array[Event]) -> Unit) -> EventListener = "m" "h"
                                ───────────┬──────────
                                           ╰──────────── Invalid stub type.

Expected behavior

All FFI stubs should compile successfully. The closure parameter is an externref on wasm-gc regardless of its internal type signature.

Environment

• moonc -v: v0.8.1+bd827dc85 (2026-02-09)
• moon version: moon 0.1.20260209 (b129ae2 2026-02-09)
• OS: Linux 6.18.7-arch1-1