# 0086 — writing to a Vector lane (`v.x = …`) panics with "unresolved type reached LLVM emission"

> **RESOLVED** (F0.5).
>
> **Root cause.** The vector-lane STORE path had no vector branch. In
> `Lowering.lowerAssignment` (`src/ir/lower.zig`) a `.field_access` target on a
> `Vector` fell through to the struct-field lookup, where no field matched a
> lane name, so `field_ty` stayed `.unresolved`. The store then built a
> `ptrTo(.unresolved)` whose pointee reached LLVM in `emitStore`
> (`src/backend/llvm/ops.zig`) → `toLLVMTypeInfo` `.unresolved` tripwire panic.
> The READ path resolved the lane fine; the two paths had diverged (issue-0083
> two-resolver class).
>
> **Fix (per file).**
> - `src/ir/lower.zig` — extracted a shared `Lowering.vectorLaneIndex(field)`
>   resolver (`.x/.y/.z/.w` + colour aliases `.r/.g/.b/.a` → lane 0..3, `null`
>   otherwise). The READ path (`lowerFieldAccessOnType`) now delegates to it
>   (dropping its silent `else 0` fallback), and a new vector branch in
>   `lowerAssignment` uses the SAME resolver to `structGepTyped` a typed pointer
>   to the lane and `storeOrCompound` with the vector element type (plain and
>   compound assignment). A non-lane field now reports a field-not-found error on
>   both paths instead of silently reading lane 0 / panicking.
> - `src/backend/llvm/ops.zig` — `emitStructGep` now addresses a vector
>   `base_type` with a `[0, lane]` `GEP2`, yielding a pointer to the lane element
>   for the scalar store.
>
> **Regression test.** `examples/1506-vectors-lane-store.sx` — `.[…]`-init and
> `= ---`-init writes, every lane of a 4-lane vector, colour aliases, and a
> compound lane assignment, reading each value back. Unit test
> `src/ir/lower.test.zig` pins the `vectorLaneIndex` contract.


## Symptom
Assigning to a component of a `Vector` local — `v.x = 1.0` (also `.y` / `.z` /
`.w`) — aborts the compiler with the internal panic:

```
thread … panic: unresolved type reached LLVM emission — a type resolution
failure was not diagnosed/aborted
  src/backend/llvm/types.zig:175  toLLVMTypeInfo  (.unresolved arm @panic)
  src/backend/llvm/ops.zig:358    emitStore       (.pointer => toLLVMType(p.pointee))
```

READING a lane (`x := v.x`) is fine; only the STORE side hits it. The init form
(`= ---` undefined vs `= .[…]` literal) does not matter — both panic once a lane
is written. A literal lane count (`Vector(3, f32)`) triggers it, so this is NOT
the lane-count resolution class (issue 0083); it is a distinct bug in the
vector-lane **store** path, where the store's pointee type resolves to the
`.unresolved` sentinel instead of the lane element type.

Discovered while fixing issue 0083 (attempt 5). It is pre-existing and orthogonal
— confirmed by reproducing on the pristine pre-0083-attempt-5 compiler — so it was
NOT introduced by the lane-count fix. The standard vector idiom (construct via a
`.[…]` literal / a constructor function returning `.[…]`, then read components or
use vector arithmetic, as in `examples/1500-vectors-vector-math.sx`) is
unaffected; only component ASSIGNMENT is broken.

## Reproduction
```sx
#import "modules/std.sx";
main :: () {
    v : Vector(3, f32) = .[0.0, 0.0, 0.0];
    v.x = 1.0;                    // panic here
    print("x={}\n", v.x);
}
```
`./zig-out/bin/sx run` panics. Removing the `v.x = 1.0` line (read-only) prints
`x=0.000000` and exits 0.

## Investigation prompt
A store to a `Vector` lane (`v.x = …`) lowers a pointer-to-lane whose pointee
type reaches LLVM as `.unresolved`, so `emitStore`
(`src/backend/llvm/ops.zig:358`, the `.pointer => toLLVMType(p.pointee)` arm)
hits the `.unresolved` tripwire panic in
`src/backend/llvm/types.zig:175`. The lane READ path computes the lane element
type correctly, so compare the lvalue/store lowering for a vector-component
assignment against the rvalue/load path — the component-write path is likely
building the lane pointer's pointee from a vector `.x`/`.y`/`.z`/`.w` field
resolution that returns `.unresolved` (or a vector field-access that resolves the
element type on load but not on store). Find where a `Vector` swizzle/component
assignment lowers its destination pointer (grep for vector component handling in
`lower.zig` assignment lowering and in the LLVM `emitStore` GEP path) and resolve
the lane element type there the same way the load path does. Verify with the
repro (expect `x=1.000000`) plus a `.[…]`-init write and a write to each of
`.x/.y/.z/.w` on a 4-lane vector, then `zig build && zig build test && bash
tests/run_examples.sh` green.