sx/issues/0151-ufcs-closure-return-pack-generic-unresolved.md

# 0151 — generic type-var not inferred through a pointer / via UFCS (LLVM SIGTRAP / "cannot infer")

## ✅ RESOLVED (2026-06-21)

**Root cause** — the generic-inference engine had no path to bind a `$T`
from a generic-struct argument head. Three gaps, all in
`src/ir/lower/generic.zig` + the UFCS dispatch:

1. `extractTypeParam` / `matchTypeParam` / `matchTypeParamStatic` lacked a
   `.parameterized_type_expr` arm — so `Box($T)` (and, recursively, the
   pointee of `*Box($T)`) never matched a type-param. Added an arm that
   recovers the arg instance's recorded per-param bindings
   (`struct_instance_bindings` + the template's ordered `type_params` via
   `struct_instance_author`) and recurses positionally.
2. The `pointer_type_expr` arm bailed when the arg wasn't itself a pointer.
   A UFCS receiver (`b.m()`) / a value passed to a `*T` param is auto-
   address-of'd, so the arg type is the *value* `Box($T)`. Added a fall-
   through that matches the pointee against the non-pointer arg.
3. `ExprTyper.inferType` had no `.lambda` arm (returned `.unresolved`), so
   the UFCS binder — which types args from the raw AST *before* they're
   lowered — couldn't read a lambda's declared return type to bind a
   `Closure(..) -> $R`. Added an arm that builds the closure type from the
   lambda's annotations.
4. A pack UFCS target (`worker: Closure(..) -> $R, ..$args`) was dispatched
   through the non-pack generic path, which can't expand the pack. Routed
   it through the SAME `lowerPackFnCall` the direct call uses, with the
   receiver spliced in as `args[0]` (a synthetic call — `lowerPackFnCall`
   reads only `call_node.args`, never the callee).

**Fix verified** — the repro prints `value=42` (both spellings). Regression
tests: `examples/0214-generics-ufcs-closure-return-pack.sx` (direct + UFCS
closure-return pack) and `examples/0215-generics-infer-through-pointer.sx`
(by-value / pointer / multi-param / nested / UFCS-auto-ref struct-head
inference). Full suite green (726/0).

**Downstream (NOT this bug):** with `await`/`cancel` now callable, the
B1.2 async examples surface a SEPARATE codegen bug — `Atomic(bool)` emits a
sub-byte (i1) atomic load/store that fails LLVM verification (filed as a new
issue). The `Future.canceled: Atomic(bool)` field hits it, so `1805`/`1806`
stay blocked on that, not on 0151.

---


## WIDENED (adversarial review of B1.2, 2026-06-21)
The UFCS-closure-return-pack case below is one symptom of a BROADER generic-inference
gap: **sx cannot infer a generic `$T` from a POINTER-wrapped argument.** Minimal repro,
no UFCS / no pack / no closure involved:
```sx
Box   :: struct ($T: Type) { v: T; }
unbox :: (b: *Box($T)) -> $T { return b.v; }
// unbox(@b)  →  error: cannot infer generic type parameter 'T' for 'unbox'
```
This blocks `await`/`cancel` in `library/modules/std/io.sx` (both take `*Future($R)`) —
they are **uncallable in EVERY form** (explicit `await(@f)` → "cannot infer 'R'"; UFCS
`f.await()` → SIGTRAP). So B1.2's async layer can CREATE a Future (`async(...)` works) but
cannot AWAIT it. Fix scope is the generic-inference engine (infer `$T` from `*T`-wrapped
params, and from closure-return-via-pack, and through UFCS dot-dispatch) — not the Io lib.
The two symptoms below + the `*Box($T)` repro above are the acceptance cases.

## Symptom

A generic free fn whose generic param `$R` is inferred from a worker
**closure's return type** — `worker: Closure(..$args) -> $R` plus a
trailing `..$args` pack — and which RETURNS a type built from `$R`
(`-> Wrap($R)`), monomorphizes correctly when called **directly**
(`mymk(recv, worker, ..)`) but leaves `$R` **unresolved** when called via
**UFCS dot syntax** (`recv.mymk(worker, ..)`). The unresolved `Wrap($R)`
reaches LLVM emission and trips the tripwire:

```
thread … panic: unresolved type reached LLVM emission — a type
resolution failure was not diagnosed/aborted
  src/backend/llvm/types.zig:180  .unresolved => @panic(...)
  … toLLVMTypeInfo (struct field) → toLLVMType
  … emit_llvm.zig:1262 declareFunction  const raw_ret_ty = self.toLLVMType(func.ret)
```

- `mymk(bx, worker, 40, 2)`  (direct)  → **works** (prints 42)
- `bx.mymk(worker, 40, 2)`    (UFCS)    → **SIGTRAP** (unresolved `$R`)

This is distinct from RESOLVED issue 0119 (UFCS generic free-fn where
`$T` is inferred from the **receiver / a slice param**). Here the
receiver does NOT carry `$R`; `$R` comes only from the **closure
return type**, and the variadic `..$args` pack is also present. The
UFCS dispatch path infers `$R` differently from (and incorrectly vs)
the direct-call path: direct resolves `$R = i64`, UFCS leaves it
`.unresolved`.

## Reproduction

Standalone — depends on no project symbols beyond `modules/std.sx`:

```sx
#import "modules/std.sx";

Box  :: struct { n: i64; }
Wrap :: struct ($R: Type) { value: R; }

mymk :: ufcs (b: Box, worker: Closure(..$args) -> $R, ..$args) -> Wrap($R) {
    f : Wrap($R) = ---;
    f.value = worker(..args);
    return f;
}

main :: () -> i32 {
    bx : Box = .{ n = 1 };
    // Direct call — works (prints 42):
    //   g := mymk(bx, (a: i64, b: i64) -> i64 => a + b, 40, 2);
    // UFCS dot-call — SIGTRAPs with "unresolved type reached LLVM emission":
    g := bx.mymk((a: i64, b: i64) -> i64 => a + b, 40, 2);
    print("value={}\n", g.value);
    return 0;
}
```

Expected: the UFCS spelling resolves `$R = i64` identically to the
direct spelling and prints `value=42`.

## Impact

Blocks Stream B1 step B1.2's verified async idiom: the user-facing
call site is `context.io.async((a,b) -> R => ..., x, y)` — a UFCS
dot-call whose `$R` is inferred from the worker closure's return type
through the `..$args` pack. The async/await LIBRARY layer (Io protocol,
`Future($R)`, blocking impl, `async`/`async_void`/`await`/`cancel`)
and the compiler plumbing (Context.io field, both `__sx_default_context`
materializers, push-inherit) are all in place and correct — the
`async` body itself works when invoked with the receiver passed
explicitly (`async(context.io, worker, ..)`). Only the UFCS dot form
hits this inference gap.

## Investigation prompt

The UFCS-vs-direct generic inference divergence lives in the UFCS
rewrite + generic-binding path. issue 0119's fix routed
`inferGenericReturnType` through the same `buildTypeBindings` the
monomorphizer uses, but that fix established bindings from the
**receiver / structured params** (`[]$T`); it does NOT cover `$R`
inferred from a **closure argument's return type** combined with a
trailing **variadic pack** on the UFCS path.

Suspect area:
- `src/ir/calls.zig` — UFCS dot-call resolution / receiver binding.
- `src/ir/lower/call.zig` — generic argument inference
  (`buildTypeBindings` / the closure-arg → `$R` binding; the
  variadic-pack `..$args` binding).
- Compare the binding map produced for the DIRECT call
  `mymk(bx, worker, 40, 2)` (resolves `$R = i64`) against the one for
  the UFCS call `bx.mymk(worker, 40, 2)` — the UFCS path must be
  dropping/mis-indexing the closure-arg's return-type contribution to
  `$R` (likely because the receiver is spliced in as arg 0 and the
  closure/pack arg indices shift, so the closure-return → `$R`
  inference rule no longer fires).

The fix likely needs: when rewriting `recv.f(args)` → `f(recv, args)`,
run the SAME closure-return-type → generic-param inference (and the
same variadic-pack binding) the direct path runs, against the
spliced-receiver argument list — so `$R` binds from the closure's
return type regardless of dot vs direct spelling.

Verification: run the repro above (`issues/0151-...sx`); expect
`value=42` (was SIGTRAP). Then restore example
`examples/1805-concurrency-io-blocking-async.sx` to the UFCS
`context.io.async(...)` form (see Stream B1 / CHECKPOINT-FIBERS) and
confirm it prints `double: 42` / `sum: 42` / `clock ok`.
```