sx/issues/0083-named-const-array-dimension-miscompiled.md

# 0083 — fixed array with a named-constant dimension is miscompiled

> **RESOLVED.** Root cause: `TypeResolver.resolveCompound`'s array arm resolved
> the dimension with `if (length.data == .int_literal) ... else 0` — a named
> const (`N :: 16`) hit the silent `else 0`, so `[N]T` became a 0-length / 0-byte
> array and element access ran out of bounds (garbage for scalars, bus error for
> slice/pointer/struct elements). Fix: the array arm now delegates the dimension
> to `inner.resolveArrayLen` (symmetric with `inner.resolveInner` for the element
> type). The stateful `Lowering.resolveArrayLen` evaluates the dimension as a
> compile-time integer across the comptime-constant, generic-value, and
> module-global const tables, and emits a diagnostic (no fabricated length) when
> it isn't one.
>
> **Exhaustive follow-up (attempt 2).** The first fix covered every *stateful*
> resolution path (direct local decls, struct fields, function params/returns),
> but the *stateless* registration-time resolver (`type_bridge`, used for type
> aliases `Arr :: [N]T` and inline union/enum field types) still resolved the
> named dim with a silent `else 0` — so `Arr :: [N]s64; a : Arr` and
> `union { a: [N]s64 }` were still miscompiled. Fix: the module-global const
> table (`ProgramIndex.module_const_map`) is now threaded into `type_bridge`
> alongside the alias map, so `StatelessInner.resolveArrayLen` resolves a named
> module-const dim to the same length everywhere. The remaining unresolvable case
> (a computed/comptime dimension on the binding-free path) bails LOUDLY instead of
> fabricating a 0 length. Files: `src/ir/type_resolver.zig`, `src/ir/lower.zig`,
> `src/ir/type_bridge.zig`. Regression: `examples/0140-types-named-const-array-dim.sx`
> (direct + type-alias + nested `[N][M]T` + union-field dims, s64 / string /
> struct element types).
>
> **Root-cause close-out (attempt 3).** Attempt 2 threaded the const map into
> `type_bridge` but the map wasn't fully populated when an alias resolved its
> dimension: type aliases (`Arr :: [N]T`) resolve EAGERLY in scanDecls pass 1,
> while TYPED consts (`N : s64 : 16`) register only in pass 2 and a
> forward-declared untyped const (`Arr :: [N]T; N :: 16`) hadn't registered yet
> either — so the stateless resolver saw an empty table, printed a non-fatal
> warning, fabricated length 0, and CONTINUED to garbage / a segfault. Three
> coordinated fixes: (1) a scanDecls **pass 0** pre-registers every integer-valued
> module const into `module_const_map` BEFORE any alias resolves, so typed,
> untyped, and forward-referenced consts all resolve identically; (2) both the
> stateful and stateless dim resolvers now share one routine
> (`program_index.moduleConstInt`) so they cannot disagree again; (3) the length-0
> fabrications are GONE — `resolveArrayLen` returns `?u32`, `resolveCompound`
> yields the `.unresolved` sentinel on null (never a 0-byte array), the stateful
> path emits a diagnostic, and the registration path surfaces an unresolved alias
> as a clean compile error that aborts the build (the `type_bridge.zig:270`
> Vector-lane `else => 0` is fixed the same way). Files:
> `src/ir/program_index.zig`, `src/ir/lower.zig`, `src/ir/type_bridge.zig`,
> `src/ir/type_resolver.zig`. Regressions:
> `examples/0143-types-typed-const-array-dim.sx` (typed-const dim direct + via
> alias for s64/string/struct, forward-ref alias, nested) and
> `examples/1129-diagnostics-array-dim-not-const.sx` (an unresolvable computed dim
> halts with a clean diagnostic + non-zero exit, not a fabricated 0-length array).
>
> **Const-expression dimensions (attempt 4).** Attempts 1–3 resolved only a BARE
> named-const dim (`[M]`) or a literal (`[5]`); any constant-FOLDABLE *expression*
> dimension (`[M + 1]`, `[M * N]`, `[N - M]`, nested `[M + N - 1]`, parenthesised
> `[(M + 1) * 2]`) was wrongly rejected as "not a compile-time integer constant"
> even though every operand is compile-time-known. Such a dimension MUST be
> evaluated, not rejected. Fix: the shared dim resolver now routes the dimension
> through a single constant integer-expression evaluator
> (`program_index.evalConstIntExpr`) that folds integer `+ - * / %` and unary
> negate (parentheses carry no AST node) over literals and named/typed module
> consts, recursively. The leaf-name lookup is delegated (`ctx.lookupDimName`) so
> the stateful body-lowering path and the stateless registration path share the
> EXACT SAME folding logic and cannot diverge — an expression dim via a type alias
> resolves identically to the direct form. The no-fabrication discipline is
> unchanged: a genuinely non-comptime dimension (a runtime local, a non-comptime
> call, an unbound name) — or arithmetic that overflows / divides by zero — still
> yields null → `.unresolved` → the same clean compile-halting diagnostic, never a
> fabricated length. Files: `src/ir/program_index.zig` (+`.test.zig`),
> `src/ir/lower.zig`, `src/ir/type_bridge.zig`. Regression:
> `examples/0144-types-const-expr-array-dim.sx` (every expression form, direct vs
> alias, scalar / string / struct element types); `1129` re-pointed at a genuinely
> non-const dimension (`[get()]s64`, a runtime call) so it still proves the
> stateless clean-halt.

## Symptom
A fixed array whose dimension is a module-global integer constant (`N :: 16;
a : [N]T`) miscompiles element access: reads/writes compute a wrong address.
With `s64` elements `a[0]` returns GARBAGE (silent); with slice/pointer element
types (`[N]string`) it Bus-errors. The identical program with a LITERAL dimension
(`a : [16]T`) is correct. Silent-miscompile class (cf. 0079–0082).

## Reproduction
```sx
#import "modules/std.sx";
N :: 16;
main :: () { a : [N]s64 = ---; a[0] = 7; print("a0={}\n", a[0]); }
```
`./zig-out/bin/sx run` prints `a0=8472789232` (garbage); want `a0=7`. Replacing
`[N]` with `[16]` prints `7`.

## Investigation prompt
A fixed-array TYPE whose dimension is a named const (`N :: 16; [N]T`) resolves to
a wrong element stride / array length in codegen — element address computation is
wrong (garbage for scalars, bad pointer for slice/pointer elements). Literal
dimensions are correct, so the defect is in resolving the array-type DIMENSION
from a constant expression (vs a literal) — the dim likely resolves to 0/unknown
or the element size is wrong. Look at array-type resolution where the length is a
const-expr (type lowering / sizeof / element-stride computation). Fix so a
named-const dimension yields the same layout as the literal. Verify with the
repro (expect 7) + a `[N]string`/`[N]struct` case (no bus error, correct reads),
and `zig build && zig build test && bash tests/run_examples.sh` green.