docs: session summary 2026-10-02 (Stage A + B/C finding)

docs/summary_2026-10-02.md

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+# Session summary — 2026-10-02 (b): Stage A + the Stage B/C finding
+
+Suite **178/178**; self-hosting fixpoint **OK** (**2,900,782 bytes**).
+Tags `v3-ast-stageA` (AST module + bug fix).  This session also
+established that the two-phase refactor is a **single** job (see §3).
+
+## Steps, tags and commits
+
+| Step | Tag | Commit | What |
+| --- | --- | --- | --- |
+| AST module | `v3-ast-stageA` | `53943fa` | `AST.def/.mod`, self-test |
+| Docs | `v3-ast-stageA` | `c91695c` | `docs/summary_ast-stageA.md` |
+| Finding | — | `7cec94a` | Stage B folds into Stage C (handoff) |
+
+## 1. Stage A — the AST module
+
+`compiler/src/AST.def` + `AST.mod`: an **arena-backed** node store,
+referenced by integer index (`Node`) like `SymTab.TypeIndex` — no
+pointers in the interface, no GC, bounded memory.
+
+- One flat `NodeRec` (`kind`, `op`, `ty`, `nch`, `child[0..7]`); every
+  construct is a kind + up to 8 children.
+- Kinds for units, declarations, type constructors, statements and
+  expressions; operator/selector codes in `op`.
+- Interned text pool (`AddTxt`/`Txt`/`TxtLen`).
+- `MakeNode`/`MakeLeaf`/`MakeBin`/`MakeUn`; `Kind`/`Op`/`Ty`/`NChild`/
+  `Child`/`AddChild`/`SetChild`; debug `Dump`.
+
+**Additive**: nothing in the grammar calls it yet.  `tests/t_ast.mod`
+builds `a := b + 2`, an unary node and a block, then walks it (exit 42);
+added to `run_tests.sh`.
+
+## 2. Bug fixed — array-of-record element stride
+
+`QbeGen.ElemSize` derived an element's stride from its **class**, and
+`ClRecord`/`ClClass`/`ClSet` fell through to **4**.  So
+
+```modula2
+TYPE R = RECORD kind, op : INTEGER; child : ARRAY [0..7] OF INTEGER END;
+VAR a : ARRAY [0..3] OF R;
+```
+
+strided by 4 instead of 48: consecutive elements overlapped and
+`a[i].op` / `a[i].child[0]` read neighbouring elements' fields.  A
+single record and a 2-element array happened to work; 4 elements did
+not (`rc=1`).  Fix: record/class/set elements use `SymTab.TypeSize`.
+Repro `rec3.mod` now exits 42.
+
+This blocked the AST self-test (its `NodeRec` holds
+`child : ARRAY [0..7] OF Node`) and is a real user-facing bug.
+
+## 3. Finding — Stage B is not separable from Stage C
+
+Starting the two-phase-declarations step (option 2) showed it is **not**
+an independent stepping stone:
+
+- Bodies are lowered **during** `ProcDecl`, interleaved with parsing.
+- To complete the symbol table before lowering a body, the body must
+  not be lowered while parsed — so it must be stored.
+- A stored body is statements + expressions, i.e. **Stage C's AST**.
+- A "declarations-only" AST does not help: the *bodies* need deferring.
+
+**Options 1 and 2 are the same job.**  The refactor must be done as one
+long branch: convert `Block`/`Stat`/`Expr` to AST nodes, move the
+`QbeGen` body calls into a lowering walk, then declarations; delete the
+forward-variable text-patching (`FwdPatchAll`/`FixLoadClass`/
+`FixStoreClass`).  Keep the old path behind a flag until the new walk
+passes the suite; expect a red fixpoint between increments.
+
+## 4. False alarm (recorded so it is not re-chased)
+
+“Nested calls mis-order arguments” — **not a bug**.  `rc` is the exit
+status modulo 256, so an encoding of `1234` observed `210` and `1274`
+observed `250`.  An equality check (`P(1,2,3,4)` → `rc=42`) confirmed
+the call and codegen are correct.
+
+## Resume
+
+```sh
+cd .../m2compiler-V3
+./bootstrap/fixpoint.sh                       # FIXPOINT OK
+cd compiler && ./build.sh && ./run_tests.sh   # 178/178
+```
+
+## Files
+
+`compiler/src/AST.def`, `compiler/src/AST.mod`,
+`compiler/src/QbeGen.mod` (`ElemSize`), `compiler/tests/t_ast.mod`,
+`compiler/run_tests.sh`, `docs/summary_ast-stageA.md`,
+`docs/session-handoff-2026-10-01.md` (revised plan), this doc.