# M2S compiles clean under V3 (compatibility additions)

Tag `v3-m2s-clean`. Main suite 110/110 (new `t_compat`).

With this, **`SymTab`, `QbeGen` and `M2S` (the Coco/R scanner) all
compile clean under V3.** `M2P`/`M2` remain (see below).

## Items knocked out (the 3 remaining, plus what they uncovered)

The three known `M2S` items — and, once those parsed, a few more
GNU-isms in CR's *generated* scanner tables:

1. **`LENGTH`** — accepted as an alias of `LEN`.
2. **Empty statement bodies** — `IF … THEN ELSIF/ELSE`, `WHILE … DO`,
   `REPEAT`, `LOOP`, `FOR` and `CASE` alternatives may now have empty
   bodies (`StatSeq` optional), matching CR's generated code.
3. **`&` = `AND`, `~` = `NOT`** — GNU/classic synonyms.
4. **Octal character literals** — new `charConst` token
   (`0C`, `03C`, `012C`) handled by `NormLit` → `CHAR` value
   (`0C` = NUL).
5. **`ORDL`** — alias of `ORD`.
6. **`CAP`** — new builtin, branchless UPCASE
   (`q := a - 32*(a>='a' AND a<='z')`, since QBE has no `select`).

Also fixed a latent bug found via `CAP`: **`ABS` (and now `CAP`) wrote
their result to a temp but `Fact` never copied it back to `q`**, so
`ABS` silently returned its operand. Both now `CopyOp(qa, q)`.

## Verification

- `tests/t_compat.mod` (exit 42) exercises `&`, `~`, `LENGTH`, `CAP`,
  `ORDL`, `0C`, and empty THEN/ELSE.
- `M2S.def` + `M2S.mod` under V3: **0 errors**.
- `SymTab` / `QbeGen` under V3: still 0 errors.
- `tools/m2make` unaffected; main suite 110/110.

## Remaining (M2P, M2)

`M2P.mod` (the Coco/R parser) is 107 errors, dominated by 102
`undeclared identifier` — **within-module forward procedure calls**
in CR's generated recursive-descent parser (gm2 hoists these; ISO
wants declaration-before-use or `FORWARD`). Also `BITSET{…}`
constructors and `symSet[0,0]` nested-array assignment. This is the
next self-hosting chunk; the driver `M2.mod` additionally needs
nested procedure values (`Error := StoreError`, `StoreError` nested in
`ListHandler`) which the current proc-type support doesn't yet
provide (static link).

## Where the compatibility additions came from

`&`, `0C`, empty bodies and forward refs are all emitted by the CR
binary's built-in scanner/parser templates — not by our `scanner.frm`
(which has none of them). So V3 accepting them is the only way to
self-host CR's output short of patching the toolchain.

## Files

`compiler/src/{M2.atg,QbeGen.def,QbeGen.mod}`,
`compiler/tests/t_compat.mod`, `compiler/run_tests.sh`.