# Session summary — SET CONST (static masks) 2026-10-18

Branch **`resolve-check`**.  Suite **225/225**; fixpoint **OK
(3,823,438 bytes)**; corpus **387/612 compile-OK, 0 crashes**.

Tag: `v3-set-constants`.

## What this session did

Landed **set constants** — the first of the two features recorded in
`docs/analysis_gaps.md` §9 — from the previous corpus sweep.

```mod
CONST Digits = CharSet{"0".."9"};          (* char range *)
      Signs  = CharSet{'+', '-'};
      NumCh  = Signs + Digits;             (* union of two set constants *)
      Warm   = CS{Red, Blue};              (* enum base *)
```

Corpus **382 → 387** (+5 files: `setconst`, `setconst2`, and 3 more),
0 crashes.  New suite tests `t_setconst` (run, exit 42) and
`t_bad_setconst` (reject, 230).

## Why it was reverted before, and the fix

A set literal is emitted as a **runtime temp** (`NewSetTemp`/`SetZero`/
`SetBit`), so a set `CONST` had no static value; accepting the
declaration produced a `230` rather than a miscompile (see the previous
sweep's finding).  Sets are also *addresses* of memory words, so the
constant must be a real `data` symbol.

The fix computes the **bit mask at parse time** — where the element
values are known and validated — and has `Lower` re-emit it as a pooled
static descriptor:

- `Lower.SetConstMaskOf` (exported) folds a set expression to
  `(mask, words, lo)`: set literals (`NkSetLit`/`NkBraceLit`), char /
  integer / named-CONST elements, `a..b` ranges, and `+`/`-`/`*`/`/`
  over two foldable set operands (union / difference / intersection /
  symdiff), plus `+ - * DIV MOD` on element ordinals.
- `ConstDecl` (grammar) calls it for `ClSet`; on success it records the
  mask (`Lower.NoteSetMask`) and lets the constant be an aggregate; on
  failure it stays a clear `230`.
- `Lower.LowerDecls` looks the mask up by name and calls the new
  `QbeGen.PoolSet`, which registers `data $setN = { w mask0, ... }` and
  returns its address.  `FlushSets` emits the pool in `EndModule`.
- The grammar's `Design` and `Lower`'s `LowerDesign` now treat a
  `ClSet` `KindConst` as an aggregate: its value *is* the descriptor
  address (so `c1 := Digits`, `c IN Digits` work).

Bound: masks are folded up to **16 words (512 bits)**; a wider `SET
CONST` stays a `230` (`MaxCFoldW`).  The corpus set constants are
≤ 8 words.

## Not fixed (recorded)

- **`setconst3.mod`** still fails: its `VAR mem: ARRAY [0..CARDINAL(
  addressLines)]` names a `CONST` declared *after* the `VAR` (a forward
  CONST reference), which is a separate gap.  Its `SET CONST` itself now
  parses.
- **Builtin calls in `CONST`** (`CAP(VAL(CHAR,65))`, `MIN`/`MAX` of
  sets) — still the second recorded feature; fold builtins over
  immediates.

## Files

`compiler/src/Lower.{def,mod}` (evaluator + `PoolSet` call),
`compiler/src/QbeGen.{def,mod}` (`PoolSet`/`FlushSets`/pool state),
`compiler/src/M2.atg` (`ConstDecl`, `Design`),
`compiler/tests/t_setconst.mod`, `compiler/tests/t_bad_setconst.mod`,
`compiler/run_tests.sh`.

## Resume

```sh
cd .../m2compiler-V3
git checkout resolve-check
cd compiler && ./build.sh && ./run_tests.sh   # 225/225
cd .. && TMP="$PWD/.scratch/fixpoint" ./bootstrap/fixpoint.sh
M2FLAGS="" ./.scratch/corpusx.sh              # 387 OK, 0 crash
```