# RESOLVED — 0126: array arg at a `[]$T` param leaves T unbound → LLVM emission panic

> **RESOLVED** (2026-06-12). Root cause: `extractTypeParam`'s
> `.slice_type_expr` arm (src/ir/lower/generic.zig) only extracted from
> `.slice`-typed args, so an array arg left `T` unbound and
> `monomorphizeFunction` stamped `.unresolved` through the body — no
> diagnostic before the emitter's sentinel panic. Fix: (1) the arm also
> extracts from `.array` args via the array's element type, mirroring
> the array→slice promotion concrete slice params perform (the existing
> coercion then handles the lowered arg); (2) `lowerGenericCall`
> (src/ir/lower/call.zig) diagnoses any still-uninferrable TYPE param
> at the call site ("cannot infer generic type parameter ...") instead
> of monomorphizing unbound — covers the deliberate string-at-`[]$T`
> gap, which used to hit the same panic. Comptime value params and
> `..$Ts` packs stay exempt. Regression tests:
> `examples/0212-generics-array-arg-slice-param.sx` (scalar/u8/struct
> elements + slice spelling; panicked or mis-typed pre-fix) and
> `examples/1168-diagnostics-generic-param-uninferrable.sx` (string
> arg; panicked pre-fix). Gates: zig build test 426/426, suite 594/594,
> distribution repo 14/14.

## Symptom

Calling a generic function whose param is a slice of the type param —
`first :: (xs: []$T) -> T` — with an ARRAY argument panics the compiler
during emission instead of compiling (or diagnosing).

- **Observed**: `panic: unresolved type reached LLVM emission — a type
  resolution failure was not diagnosed/aborted`
  (src/backend/llvm/types.zig:175, via `emitIndexGet` in the
  monomorphized body).
- **Expected**: the array coerces to a slice at the `[]T` param — the
  same promotion a CONCRETE `[]i64` param (and a `[]i64`-annotated
  local) already performs — so `T` binds from the array's element type
  and the call compiles.

Passing an actual slice works (`s : []i64 = a; first(s)` prints the
element); only the direct array spelling breaks, and only for generic
slice params.

## Reproduction

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

first :: (xs: []$T) -> T {
    return xs[0];
}

main :: () -> i32 {
    a : [3]i64 = ---;
    a[0] = 7; a[1] = 8; a[2] = 9;
    v := first(a);
    print("{}\n", v);
    return 0;
}
```

Observed at master 837b5d3: the panic above. With `s : []i64 = a;
first(s)` it prints `7`.

## Investigation prompt

Root cause: `extractTypeParam` (src/ir/lower/generic.zig, the
`.slice_type_expr` arm) only extracts when the ARG type is itself a
`.slice` — an `.array` arg returns null, so `buildTypeBindings` leaves
`T` unbound, `monomorphizeFunction` stamps the body with `T →
.unresolved`, and nothing diagnoses before the emitter's sentinel
tripwire fires (the declaration-time gate from ca5bd52 doesn't apply:
this `T` IS bindable — the call-site inference is what failed).

Fix: mirror the existing array→slice param coercion in the binding
extractor — in the `.slice_type_expr` arm, when the arg type is an
`.array`, recurse on the array's ELEMENT type exactly as the `.slice`
case does. Verify the lowered call then coerces the array arg to the
mono's now-concrete `[]T` param (the same `array_to_slice` the
concrete path uses) — if not, the generic dispatch arg path needs the
same promotion.

Out of scope, known gap (CHECKPOINT-MEM): `string` deliberately does
not bind `[]$T` — that case diagnoses "unknown type 'T'" and its
story is deferred to the mem-stream phases.

Verification: the repro prints `7`; the slice spelling still works;
`zig build && zig build test`, `bash tests/run_examples.sh` green;
pin the repro as a generics example.
