# 0087 - oversized compile-time integer in type dimension/lane panics

> **RESOLVED.** Root cause: an array dimension / Vector lane folded to a valid
> `i64` (e.g. `5_000_000_000`) was then narrowed to `u32` with an unchecked
> `@intCast` at three sites (`Lowering.resolveArrayLen` lower.zig:11656,
> `Lowering.resolveVectorLane` lower.zig:11836, `StatelessInner.resolveArrayLen`
> type_bridge.zig:58), aborting the COMPILER with "integer does not fit in
> destination type" — the fold was correct, the narrowing was not. Fix: a single
> range-checked fold-to-u32 gate, `program_index.foldDimU32(node, ctx, min)`,
> folds via `evalConstIntExpr` then checks `[min, maxInt(u32)]` and returns a
> tagged `DimU32` (`.ok` / `.not_const` / `.below_min` / `.too_large`). Every
> dim/lane narrowing now routes through it — no call site does a bare `@intCast`,
> so an out-of-u32-range dim/lane surfaces a clean diagnostic ("array dimension N
> does not fit in u32" / "Vector lane count N does not fit in u32") and halts the
> build (exit 1) instead of panicking. Value-param args stay `i64` until used as
> a dim/lane, where the same gate checks them. Files: `src/ir/program_index.zig`
> (`DimU32` + `foldDimU32`), `src/ir/lower.zig`, `src/ir/type_bridge.zig`.
> Regressions: `examples/1130-diagnostics-array-dim-oversized-u32.sx` (oversized
> array dim → clean halt) and `examples/1503-vectors-oversized-lane-not-u32.sx`
> (oversized Vector lane → clean halt).

## Symptom
An oversized compile-time integer used where the compiler expects a `u32`
array dimension or Vector lane count panics inside the compiler instead of
emitting a source diagnostic.

Observed: `@intCast` panics with "integer does not fit in destination type" in
`Lowering.resolveArrayLen` / `Lowering.resolveVectorLane`.

Expected: compile halts with a normal diagnostic such as "array dimension does
not fit in u32" / "Vector lane count must fit in u32", and no compiler panic.

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

main :: () {
    a : [5000000000]i64 = ---;
    print("{}\n", a.len);
}
```

Vector lane sibling:

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

main :: () {
    v : Vector(5000000000, f32) = .[];
    print("{}\n", v);
}
```

## Investigation prompt
Fix oversized compile-time integer handling for fixed-array dimensions and
Vector lane counts. Suspected area: `src/ir/lower.zig`
`Lowering.resolveArrayLen` and `Lowering.resolveVectorLane`, plus the stateless
adapter in `src/ir/type_bridge.zig` `StatelessInner.resolveArrayLen`. These
functions fold to `i64` through `program_index.evalConstIntExpr`, then cast to
`u32` with `@intCast`; values greater than `std.math.maxInt(u32)` panic in the
compiler. The fix likely needs an explicit range check before every `u32` cast,
with a diagnostic on the stateful path and null / `.unresolved` propagation on
the stateless path. Verify both repros exit 1 with source diagnostics and no
panic, then run `zig build && zig build test && bash tests/run_examples.sh`.
