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

0151 — UFCS dot-call: $R inferred from a closure's RETURN type via a variadic pack is left unresolved (LLVM SIGTRAP)

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:

#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.