# Fix — nested-array elements (double load → wrong code / signal)

Tag `v3-fix-nested-arrays`. Main suite 108/108; `QbeGen` still compiles
clean under V3; `tools/m2make` 18/18.

## Symptom

Two failures surfaced by `M2make` (documented in
`tools/m2make/README.md`), which forced its portable core onto a
"flat 1-D arrays only" subset:

- passing a nested-array element (a row) to an open-array formal,
  e.g. `Sum(m[1])`, **segfaulted**;
- the `a[i][j]` / `HIGH(a[i])` patterns in `M2makeOS` could not be used.

## Root cause

`Fact`'s `Design` branch loaded array-typed designators a second time.
`Design` already materialises the array value for array-typed suffixes:
the `[i]` index handler and the `.field` handler both emit `ElemLoad`
(load the sub-descriptor pointer) when the element type is an array.
`Fact` then did `ElemLoad` again, so the row pointer was replaced by its
first word (the element count), which was passed on as an address:

```
%t129 =l loadl %t128     ; row descriptor address (correct, from Design)
%t130 =l loadl %t129     ; extra load: dereferences the row -> count
call $Sum_0(l 0, l %t130)
```

## Fix

`compiler/src/M2.atg` (`Fact`, `Design` branch): treat `ClArray` like
`ClRecord` / `ClSet` / `ClClass` — an array designator's value *is* its
address, so `CopyOp(qd, q)` instead of `ElemLoad(qd, dt, q)`.

## Verification

- `Sum(m[1])` went from SIGSEGV (139) to the expected value (15),
  confirmed by reverting and re-applying the one-line change.
- New regression test `compiler/tests/t_nestidx.mod` (exit 42) covers
  `a[i][j]` loads **and** stores, the comma form `a[i, j]`,
  `HIGH(row)` via `Sum`, and passing rows to an open-array formal.
- The `M2makeOS` pattern (`argTab[i][k]`, `HIGH(argTab[i])`,
  `CopyArg(s, argTab[argN])`) now compiles and runs.
- `QbeGen.def/.mod` still compile clean under V3 (0 errors).

## Note on the "233 on `a[i][j]` loads"

A minimal `a[i][j]` load/store parsed and ran correctly even with the
fix reverted, so no separate rejection was reproducible; the
open-array-actual case is the one that definitely miscompiled. The
one-line change above is the common fix for the nested-array-element
value path.

## Files

`compiler/src/M2.atg`, `compiler/tests/t_nestidx.mod`,
`compiler/run_tests.sh`.