# Lower phase — L4 and L5 status (2026-10-05)

Branch **`ast-stage-c`**.  All slices below are byte-verified
(`cmp gen_ssa/<Mod>.ssa gen_ssa/<Mod>L.ssa`), gated by the suite and the
self-hosting fixpoint.

## Landed

| Tag | Commit | Content |
| --- | --- | --- |
| `v3-lower4` | `53a1d15` | builtins, `CASE`, `WITH` |
| `v3-lower5-core` | `ab507e2` | records, arrays, designators, aggregate copies |
| `v3-lower5-lit` | `5e23bfe` | char/real/string/set literals, `IN`, typed relations |
| `v3-lower5-sets` | `081869f` | set arithmetic, string concatenation |
| `v3-lower5-params` | `0b22c8a` | aggregate formals, value open arrays |

Suite **191/191**; `FIXPOINT OK` (stage2 == stage3, 3,304,549 bytes).

## Design

- **Unified walk.**  `LowerExprFull(n, ..., asAddr)` handles both value
  and address contexts; `LowerExpr` is the value wrapper and
  `LowerDesign` the address wrapper.  Selectors (`.field` / `[i]` / `^`)
  are walked once, then `FinalLoad` applies Fact's trailing
  scalar/pointer load (aggregates keep the address).
- **Gate.**  `CanType`/`CanDesign`/`CanCaseLabel`/`CanBuiltin` mirror the
  walk's supported subset; `CanLower` accepts TYPE declarations and
  aggregate VAR/proc types.
- **Builtins** dispatch by callee name (before user calls):
  `ABS CAP CHR ORD SIZE/TSIZE MIN/MAX SHIFT ROTATE ADR`,
  statements `INC/DEC INCL/EXCL NEW/DISPOSE`.
- **CASE** is a compare-chain mirroring `CaseStat/CaseAlt/CaseLabel`
  (value and `lo..hi` labels, ELSE arm).
- **WITH** uses a `withRec`/`withTop` stack shared by the gate and the
  walk; bare field idents resolve through `WithField` + `QbeGen.TopWith`.
- **Relations** dispatch on operand type: `IN`, pointer (`CmpL`), set
  (`CmpSet`), string (`StrEq`), long (`CmpLong`), else `Cmp`.
- **Arithmetic** covers set `+ - * /` (`SetBinOp`) and string `+`
  (`StrFold`/`StrCat`), plus proper long/real result types.
- **Open arrays**: a value `ARRAY OF T` formal is passed by descriptor
  address (`FuncParam(..., isV OR IsOpenArray(t))`), matching the
  grammar.

## Tests

`t_lower4` (builtins/sets/pointers), `t_lower4case`, `t_lower4with`,
`t_lower5` (records/arrays), `t_lower5arr` (open-array + VAR record
params), `t_lower5lit` (literals/IN), `t_lower5set` (set arithmetic +
string concat).

## Remaining (documented, not blocking)

- **UString** literals/ops.
- **Aggregate constants** (record/array CONST values).
- **Qualified names / imports / class methods** — L6 territory.

## Fixed after L5

- **Nested arrays** (`ARRAY OF ARRAY` record fields): `QbeGen.ArrBodyItems`
  now expands array-of-record elements per element (nested `EmitRec`)
  when the record recursively has a nested-array field, and `ArrData`
  emits the matching per-element sub-descriptors (nested
  `EmitRecStatics`).  Plain record arrays stay a compact zero blob, so
  `AST.arena` does not balloon.  Regression: `tests/t_nestarr.mod`
  (was `docs/wip/nested-array-field-bug.mod`).  Suite 192/192;
  FIXPOINT OK (3,322,779 bytes).

## Not touched

`QbeGen` remains at its committed state (stride fix only); the general
array-of-record array-field fix stays deferred
(`docs/summary_l3_selfhost_fix.md`).