# 0094 — assigning to a missing struct field panics with "unresolved type reached LLVM emission"

> **RESOLVED** (F0.10). **Root cause:** the lvalue field lookup never diagnosed a
> missing field. In `Lowering.lowerAssignment`'s `.field_access` target path
> (`src/ir/lower.zig`), `field_ty` started as `.unresolved`; when no struct field
> matched, the code still built `ptrTo(field_ty)` / `structGepTyped` and stored —
> so a pointer-to-`.unresolved` reached LLVM emission and tripped the
> `src/backend/llvm/types.zig` tripwire. The nested lvalue-pointer path
> (`Lowering.lowerExprAsPtr`'s `.field_access` fallback) had the sibling defect:
> on a miss it returned `structGepTyped(obj_ptr, 0, .s64, obj_ty)` — a silent
> field-0/`.s64` default.
>
> **Fix (`src/ir/lower.zig`):** all three lvalue field-store sites — single
> assignment, address-of, and multi-target assignment — route field resolution
> through one shared helper, `fieldLvaluePtr(obj_ptr, obj_ty, field)`, which
> resolves struct fields, union/tagged-union direct fields, promoted
> anonymous-struct union members, and tuple elements, and returns `null` (no
> field 0 / `.unresolved` default) when nothing matches. Each caller emits the
> read path's field-not-found diagnostic (`emitFieldError`) on a `null` result:
> 1. `lowerAssignment` `.field_access` target — `found`-flag bail on the struct
>    loop, with union direct + promoted handled before it.
> 2. `lowerExprAsPtr` `.field_access` — delegates to `fieldLvaluePtr`, so the
>    address-of path now resolves promoted union members (`@v.x`) — not only
>    direct union fields — and a genuine miss errors. The `.s64` sentinel is gone.
> 3. `lowerMultiAssign` `.field_access` target — replaced its struct-only loop
>    (which defaulted `field_idx 0` / `field_ty .unresolved` on a miss, silently
>    storing into field 0 — `p.q, y = 2, 3` printed `x=2 y=3`) with the shared
>    `fieldLvaluePtr`; a missing field now errors, and a valid promoted-union /
>    tuple member at a non-zero offset stores into its own slot, not field 0.
>
> All sites reuse `emitFieldError` (the exact facility the read path
> `lowerFieldAccessOnType` uses), so the read and write paths reject identically.
> The `src/backend/llvm/types.zig` tripwire is untouched — the fix is to never
> produce `.unresolved` for a missing-field store.
>
> **Regression tests:** `examples/1145-diagnostics-missing-struct-field-assign.sx`
> (negative — single-assign, address-of, AND multi-assign missing-field all error,
> exit 1), `examples/0165-types-nested-struct-field-assign.sx` (positive — nested
> struct field write + address-of a matched field), `examples/0166-types-union-promoted-member-lvalue.sx`
> (positive — promoted union member written and address-of'd, including a non-zero
> offset member), and two lowering unit tests in `src/ir/lower.test.zig`
> (single- and multi-assign missing-field field-not-found).

## Symptom
Assigning to a nonexistent struct field (`p.q = ...`) panics during LLVM emission instead of reporting a source diagnostic.

Observed: the compiler reaches the `.unresolved` LLVM tripwire in `src/backend/llvm/types.zig:175` via `emitStore`.
Expected: a normal compile error like `field 'q' not found on type 'Point'`, matching the read-field diagnostic path.

## Reproduction
```sx
Point :: struct { x: s64; }

main :: () {
    p := Point.{ x = 1 };
    p.q = 2;
}
```

Running `./zig-out/bin/sx run repro.sx` currently panics with:
```text
panic: unresolved type reached LLVM emission — a type resolution failure was not diagnosed/aborted
```

## Investigation prompt
Fix issue 0094 in the sx compiler: assigning to a missing struct field (`p.q = 2`) panics with `.unresolved` reaching LLVM emission instead of emitting a field-not-found diagnostic.

Suspected area: `src/ir/lower.zig`, especially `Lowering.lowerAssignment`'s `.field_access` target path around the struct-field lookup (`field_ty` starts as `.unresolved`, no matched field diagnoses, then `ptrTo(field_ty)` is stored) and the related `Lowering.lowerExprAsPtr` field-access fallback that returns `structGepTyped(obj_ptr, 0, .s64, obj_ty)` on lookup failure. The fix should make failed lvalue field lookup loud, reusing `emitFieldError(obj_ty, field, span)` or equivalent, and should not use `.s64`, `.void`, or any real type as a sentinel.

Verification: run the repro and expect exit 1 with a source diagnostic `field 'q' not found on type 'Point'`; no LLVM panic. Then run `zig build`, `zig build test`, and `bash tests/run_examples.sh`.