# Step: variant records

Tag `v3-variant-records`. Suite **132/132**; fixpoint **OK**
(image **2,135,590 bytes**).

## Grammar

```
RECORD
  id : INTEGER;                 (* plain fields *)
; CASE k : Kind OF              (* the variant part *)
    circle : radius : INTEGER;
  | square : side : INTEGER;
  | rect   : w, h : INTEGER;
  END
END
```

- `RecordType` = `RECORD [ RecItem { ";" [ RecItem ] } ] END`, where
  `RecItem` is a plain `RecField` or a `CaseField`. Because a variant
  part starts with the `CASE` keyword it is unambiguously
  distinguishable from a field (which starts with an identifier), so
  the grammar stays LL(1).
- `CaseField` = `CASE tag : Type OF RecFieldList { "|" RecFieldList } END`.
  Each `RecFieldList` is `VarLabel [ ".." VarLabel ] ":" RecField …`;
  labels are parsed but not interpreted (the layout overlays
  regardless).
- The tag must be an ordinal type (else error **224**).

## Layout (`SymTab`)

- Fields are registered per record as before; the tag field is recorded
  via `SetVariantTag`, and the record is flagged with `MarkVariant`.
- `ComputeOffsets`: fields declared **before** the tag pack normally;
  the tag gets the next slot; **every** branch field overlays in the
  region after the tag. (Ranks are reverse-declaration, so "before the
  tag" is `ord < tag^.ord`.)
- `TypeSizeD`: `plainSize + tagSize + max(branch field sizes)`.

Example (`Kind`, then `Shape`): `id`@0, `k`@4, `radius`/`side`/`w`,`h`
all @8; size 12. Branch fields share storage, so assigning `w` then
`h` leaves `h`, and switching `k` reuses the slot — exactly the
variant-record contract.

`ResetFields` clears a record's pending-field buffer and layout at each
`|` so later branches re-register; field names stay visible for
`WITH`/`.field` access.

## Notes

- Whole-record equality/assignment for a variant record is not offered
  (overlapping storage makes a field-by-field copy wrong); the existing
  array/record strictness gives 213 for such comparisons.
- `WITH` over a variant's fields works (verified).

## Tests

`t_variant.mod` (exit 31): plain field + tag + three branches, overlay
(`w`/`h` share), branch switch (`square` reuses the slot).

## Files

`compiler/src/M2.atg` (`RecItem`/`CaseField`/`VarLabel`,
`RecFieldList`), `compiler/src/SymTab.def`/`.mod`
(`IsOrdinal`, `SetVariantTag`, `MarkVariant`, `IsVariant`,
`ResetFields`, `ComputeOffsets`, `TypeSizeD`),
`compiler/src/compiler.frm` (message 236 `bad variant record`),
`compiler/tests/t_variant.mod`, `compiler/run_tests.sh`,
`docs/features.md`.