# Step: Clarion CLASS lowering

Tag `v3-class-lowering`. Suite **141/141**; fixpoint **OK**
(image **2,188,065 bytes**).

## What "CLASS lowering" means

Clarion (a 1990s PC Modula-2 dialect) added OOP to classic
Modula-2 — the syntax is preserved in `docs/OOP.txt`. "Lowering" is
translating a high-level construct into what the backend already
understands. Previously V3 *parsed and checked* classes but ended
every one in error 230 and emitted no method bodies ("declared +
checked only"). This step makes **non-virtual** classes fully
executable:

```modula-2
TYPE
  CLASS Point;
    x, y : INTEGER;
    PROCEDURE Set(a, b : INTEGER);
    PROCEDURE Sum() : INTEGER;
  END Point;
CLASS IMPLEMENTATION Point;
  PROCEDURE Set(a, b : INTEGER); BEGIN x := a; y := b END Set;
  PROCEDURE Sum() : INTEGER; BEGIN RETURN x + y END Sum;
END Point;
VAR p : Point;
BEGIN p.Set(3, 4); ExitCode := p.Sum() END   (* -> 7 *)
```

## How it works

1. **Layout** — a class is a record: `ComputeOffsets` gives fields
   declaration-order offsets. Single inheritance lays the child's own
   fields after the parent's (`baseOff`), and `FindField` /
   `FieldExists` / `FieldOffset` walk up the parent chain, so
   `obj.field` works through the existing record path.
2. **Receiver (`THIS`)** — each method gets a **hidden first `VAR`
   parameter `THIS`** of the class type. At the top of the body it is
   pushed as the current `WITH` base, so a bare field name resolves
   to `THIS^.field` — reusing all the `WITH` addressing machinery.
   (`SymTab.EnterThisParam`, `QbeGen.ThisBase`/`PushWith`.)
3. **Calls** — `obj.M(args)` binds statically: `Design` recognises a
   class method, arms the receiver (`QbeGen.ArmRecv`), and `ArgList`
   emits the call with `obj`'s address as the hidden first argument
   (`CallBegin` prepends the armed receiver). Formal types come from
   `ClassMethodParamType`/`…IsVar` (user index 0 skips `THIS`).
4. **Implementation binding** — `ResumeMethod` reuses the method
   symbol declared in the class scope (so the call-site uid matches
   the definition's, and `MethUid` reports it since the class scope
   is detached by then).
5. **`VIRTUAL`** is accepted and **dispatched statically** for now
   (no vtable yet).

## Supported / not yet

- ✅ Fields (`obj.field`, `WITH obj`, inherited fields), methods
  (value + `VAR` formals, value-returning), single inheritance
  (inherited fields and methods; a child method sees both), classes
  in a `DEFINITION`/`IMPLEMENTATION` pair.
- ⏸ **Dispatch** for `VIRTUAL` (currently static).
- ⏸ **Bare sibling-method calls** inside a method body
  (`Add(a)` rather than `obj.Add(a)`); call through an object.
- ⏸ A class's `BEGIN … END` initialisation body (still 230).
- ⏸ Multiple parents (still 230, as designed).

## Implementation notes

- V3 is **single-pass** (declaration before use), so the class
  helpers had to be ordered: `Resolve`/`ParentOf`/`PushProc` early,
  `LayoutClass` after `ComputeOffsets`, `EnterThisParam` after
  `EnterParam`. (gm2 tolerates forward references; the fixpoint's V3
  stage is the guard.)
- `MethodImpl` now emits the body: `EndFuncHeader` (was missing, so
  bodies leaked to top level), the receiver binding, `Block`, then
  `PopWith`/`CloseProc`/`EndFunc`.
- `EnterThisParam` clears the pending-formal buffer so `THIS` isn't
  emitted twice by `ParamSection`.

## Tests

- `t_classmethod.mod` (7): fields via the receiver, value + `VAR`
  formals, value-returning method, `VIRTUAL` static dispatch.
- `t_classinherit.mod` (7): inherited method + inherited field seen
  from a child method.
- `TClassDefProg` (8): a class declared in a DEFINITION used as data.
- `t_class.mod` still rejects (its `BEGIN` init body is 230).

## Files

`compiler/src/SymTab.def`/`.mod` (parent field inheritance,
`ParentOf`/`EnterFieldsOf`, `EnterThisParam`, `ResumeMethod`,
`MethUid`, `ClassMethod*`, `LayoutClass`),
`compiler/src/QbeGen.def`/`.mod` (`ThisBase`, `ArmRecv`, receiver
argument in `CallBegin`), `compiler/src/M2.atg` (`ClassRest`/
`ClassImplRest`/`MethodImpl`, `ProcHeading` receiver + method resume,
`Design` method branch, `ActParam`/`ArgList` method formals),
`compiler/tests/{t_classmethod,t_classinherit,t_classdef_prog}.mod`,
`docs/features.md`.