sx/issues/0153-reexport-generic-value-failable-loses-error-channel.md

# 0153 — a re-exported generic value-failable `($R, !E)` loses its `!` error channel

## ✅ RESOLVED (2026-06-21)

**Root cause** — `GenericResolver.inferGenericReturnType`
(`src/ir/generics.zig`) resolved the generic call's return-type AST
(`($R, !E)`) in the CALL-SITE module context. For a re-exported fn the error
set name (`LE` / `IoErr`, re-exported as `LE :: lib.LE`) resolved through the
call-site alias to a TypeId that is NOT tagged `.error_set`, so the planned
result was a tuple whose last field wasn't an error set — `errorChannelOf`
(`lower/error.zig:148`) saw a plain tuple and the failable channel was lost.
`monomorphizeFunction` already pins the source to the fn's defining module
before resolving the return type; `inferGenericReturnType` did not, so the
planned call-result type and the instance's real signature disagreed.

**Fix** — pin the source to the function's defining module
(`fd.body.source_file`) around the return-type resolution in
`inferGenericReturnType`, mirroring `monomorphizeFunction`. The binding-build
stays in the call-site context (its args are typed there). Now the `!E`
resolves to the same `.error_set` TypeId the instance's signature uses.

**Verified** — the repro prints `r=42`; regression test
`examples/1058-errors-reexport-value-failable-channel.sx` (+ companion
`lib.sx`). This also unblocked the B1.2 async surface end-to-end:
`examples/1805-concurrency-io-blocking-async.sx` (`sum: 42` / `double: 42` /
`clock ok`) + `examples/1806-concurrency-io-cancel.sx` (cancel → `await`
raises `.Canceled`). Full suite green.

---


## Symptom

A generic function returning a value-failable `($R, !E)` keeps its error
channel when called from the module that declares it, but **loses it when
the function is reached through a re-export alias** (`get :: lib.get;`). At
the consumer the call result is typed as a plain **tuple** (last field is a
*non*-`.error_set` type), so:

- `try f()` → `error: `try` requires a failable expression; operand has type 'tuple'`
- `f() or { default }` → LLVM verification failure — the `or` merge PHI is
  typed `i1` (the lost-channel discriminant) but carries the `i64` default:
  ```
  PHI node operands are not the same type as the result!
    %bp = phi i1 [ true, %entry ], [ -1, %or.rhs.0 ]
  ```

It requires **both** conditions — drop either and it works:

- non-generic re-exported value-failable + `or` → **works**
- generic value-failable imported **directly** (no re-export) + `or` → **works**
- generic value-failable, **direct call** (no UFCS) through a re-export → **fails** too
  (so it is NOT UFCS-specific)

## Reproduction

Co-located minimal repro (two files, no project deps beyond `modules/std.sx`):
`issues/0153-...sx` (consumer) + `issues/0153-.../lib.sx` (impl). Run the
consumer; expect `r=42`, get the PHI verification failure.

```sx
// lib.sx
#import "modules/std.sx";
LE  :: error { Bad }
Box :: struct ($R: Type) { v: R; }
get :: ufcs (b: *Box($R)) -> ($R, !LE) { return b.v; }
```
```sx
// main
#import "modules/std.sx";
lib :: #import ".../lib.sx";
Box :: lib.Box;            // re-export the generic struct,
get :: lib.get;            // the generic value-failable fn,
LE  :: lib.LE;             // AND its error set (the std.sx facade pattern)
main :: () -> i32 {
    b : Box(i64) = .{ v = 42 };
    r := b.get() or { -1 };       // ← PHI i1/i64 mismatch
    print("r={}\n", r);
    return 0;
}
```

Real-world one-liner (same bug, via the stdlib facade — `await`/`IoErr` are
re-exported from `std/io.sx` through `std.sx`):

```sx
#import "modules/std.sx";
#import "modules/std/atomic.sx";
main :: () -> i32 {
    f : Future(i64) = ---;
    f.value = 42; f.state = .ready; f.canceled = Atomic(bool).init(false);
    r := f.await() or { -1 };     // ← same PHI mismatch
    print("r={}\n", r);
    return 0;
}
```

## Impact

Blocks the B1.2 async surface (the LAST blocker after 0151 + 0152). `await`
returns `($R, !IoErr)` and is re-exported via `std.sx`
(`await :: io_mod.await; IoErr :: io_mod.IoErr;`), so every
`context.io.async(...).await() or { … }` / `try …await()` hits this. The
async runtime itself is correct (Futures build, `$R` infers, the value is
right) — only the call-site failable typing is wrong.

## Investigation prompt

A value-failable `(T, !E)` is represented as a **tuple whose LAST field is
an `.error_set` TypeId** — that is exactly what `Lowering.errorChannelOf`
(`src/ir/lower/error.zig:148`) keys on. The bug is that the call-result type
inferred for a re-exported generic fn is a tuple whose last field is NOT an
`.error_set`, so `errorChannelOf` returns null (→ "plain tuple").

Suspect: the generic return-type resolution
(`inferGenericReturnType` / `buildTypeBindings` in `src/ir/generics.zig` +
`monomorphizeFunction` in `src/ir/lower/generic.zig`) resolves the fn's
return-type AST `($R, !LE)` in a module context where the error-set name
reached through the re-export alias (`LE :: lib.LE`) resolves to a TypeId
that is NOT tagged `.error_set` (a duplicate/plain interning of the aliased
error type, or the alias is followed to a non-error-set placeholder). The
"generic + re-export" co-requirement points at the monomorphized return-type
path specifically — a non-generic re-export keeps the channel (its return
type isn't re-resolved per-instance), and a direct generic import keeps it
(the error set resolves in its own module).

Steps:
1. At the consumer call site, dump the inferred call-result TypeId for
   `b.get()` and inspect its last tuple field's `TypeInfo` — confirm it is
   NOT `.error_set` (vs the direct-import case, where it IS).
2. Trace where the aliased error-set name (`LE` / `IoErr`) is resolved during
   the instance's return-type construction; ensure it resolves to the SAME
   `.error_set` TypeId the declaring module interned (follow the re-export
   alias to the original error set, don't re-intern a plain type).

Verification: run the co-located repro; expect `r=42`. Then restore the B1.2
async examples (`examples/1805-concurrency-io-blocking-async.sx` +
`1806-...-io-cancel.sx`) per CHECKPOINT-FIBERS and confirm
`sum: 42` / `double: 42` / cancel raises `.Canceled`.