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.
>
> **Unified comptime-int evaluator (attempt 5).** Attempts 1–4 fixed the array
> *dimension* paths but the SAME length-0 fabrication class survived on the
> siblings that resolve a comptime integer elsewhere: the three Vector lane
> resolvers (`resolveTypeCallWithBindings`, `resolveParameterizedWithBindings`,
> `resolveArrayLiteralType`) and the two generic value-param binders
> (`instantiateGenericStruct`, `instantiateTypeFunction`) each hand-rolled an
> `else => 0` switch, so `Vector(N, f32)` / `Vec(N, f32)` (N a module const)
> fabricated a 0-lane `<0 x float>` (LLVM "huge alignment" abort) or a 0 binding
> under a wrong mangled name; and the `inline for` bound folder (`evalComptimeInt`)
> only knew literals / comptime cursors / `<pack>.len`, so `inline for 0..M` failed
> outright. Fix: every one of those sites now routes through the single shared
> `program_index.evalConstIntExpr` — `evalComptimeInt` delegates to it (the pack
> `.len` leaf moved into the shared folder via a new `ctx.lookupPackLen`); the
> Vector lane and value-param resolvers fold through it and emit a clean diagnostic
> + `.unresolved` (never `else => 0`) on a non-const operand. Two enabling fixes
> upstream of resolution: the unknown-type semantic checker no longer walks a
> value-param position (`Vector(N, …)` / `Vec(N, …)`) as a type name (it was
> reporting "unknown type 'N'"); and both the parameterized-type-arg parser and
> the function-body-detection lookahead (`hasFnBodyAfterArrow`) accept a
> const-EXPRESSION in a value position, so `Vector(M + 1, f32)` and `[M + 1]T`
> parse as a return type too (the latter a pre-existing attempt-4 sibling miss).
> Files: `src/ir/program_index.zig` (+`.test.zig`), `src/ir/lower.zig`,
> `src/ir/type_bridge.zig`, `src/ir/semantic_diagnostics.zig`, `src/parser.zig`.
> Regressions: `examples/1501-vectors-const-lane.sx` (named-const + const-expr
> lane, direct + alias, 3- and 4-lane reads), `examples/1502-vectors-runtime-lane-
> not-const.sx` (a runtime lane clean-halts, exit 1, no LLVM crash),
> `examples/0207-generics-value-param-const.sx` (`Vec(N,f32)` / `Vec(M+1,f32)`
> resolve to the same instantiation as `Vec(3,f32)`),
> `examples/0610-comptime-inline-for-const-bound.sx` (`inline for 0..M` and
> `0..(M+1)` unroll).
>
> **Value-param type functions + oversized guard (attempt 6).** Two remaining
> siblings in the comptime-int path. (1) A type-RETURNING function with a value
> param used as a TYPE annotation (`b : Make(N, s64)` where `Make :: ($K: u32,
> $T: Type) -> Type { return [K]T; }`) was rejected "unknown type 'N'" because
> the unknown-type checker walked the value-param position as a type name, AND the
> parameterized-type-annotation path never routed to `instantiateTypeFunction`
> (only the `.call` path did), nor did that binder resolve a non-struct/union
> return shape. Fix: `isValueParamPosition` (semantic_diagnostics.zig) now also
> skips a value param of a `fn_ast_map` type-returning function (mirroring the
> binder's value/type classification); `resolveParameterizedWithBindings` routes
> a type-returning-function name to `instantiateTypeFunction`; and that binder
> resolves a general return-type expression (`return [K]T`) with bindings active.
> `Make(N, s64)`, `Make(M + 1, s64)`, and `Make(3, s64)` now resolve to one
> `[3]s64`. (2) Oversized dim/lane folds (`[5_000_000_000]`) panicked the
> compiler — fixed under issue 0087 via the shared range-checked
> `program_index.foldDimU32` gate. Files: `src/ir/semantic_diagnostics.zig`,
> `src/ir/lower.zig`, `src/ir/program_index.zig`, `src/ir/type_bridge.zig`.
> Regression: `examples/0208-generics-value-param-type-function.sx`.
>
> **Diagnostic-accuracy parity (attempt 7).** The fold + layout were correct, but
> the two paths still DIVERGED on the error MESSAGE for an oversized dim. The
> direct form (`a : [5_000_000_000]s64`) reported the accurate "array dimension
> 5000000000 does not fit in u32" (from the stateful `resolveArrayLen`, which
> branches on `foldDimU32`'s `.too_large` / `.below_min` / `.not_const` variants),
> but the type-ALIAS form (`Big :: [5_000_000_000]s64`) reported a FALSE "an array
> dimension is not a compile-time integer constant" — because the stateless
> `resolveArrayLen` collapsed every non-`.ok` `DimU32` to `null`, so the
> alias-registration site had only one generic message to emit. Fix: a single
> wording source `program_index.reportDimError(diag, span, DimU32)` now owns the
> dim-error text; the stateful path emits through it, and the alias-registration
> site re-folds a top-level array dim via the new `type_bridge.foldArrayDim`
> (same shared `foldDimU32`) and routes a `.too_large` / `.below_min` result to
> `reportDimError` — so an oversized alias dim now reports the SAME precise
> message as the direct form. A genuinely non-const alias dim (`[get()]`) still
> gets the alias-specific "not a compile-time integer constant" message (1129).
> Files: `src/ir/program_index.zig`, `src/ir/type_bridge.zig`, `src/ir/lower.zig`.
> Regression: `examples/1131-diagnostics-array-dim-oversized-u32-alias.sx`
> (oversized dim via alias → "does not fit in u32", matching direct example 1130;
> 1129 still proves the non-const path keeps the generic message).

## 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.