docs(specs): add Index Operator design; CURRENT_TASK: plan for Phase‑20.31 small-scope bring‑up

CURRENT_TASK.md

@@ -136,6 +136,29 @@ Plan — Next（一本化の続きと段階導入）
   - 型注釈の最小拡張（観測ベースで1〜2件）。
   - phase‑15.7/README と Quick Reference に「内部正規化（obj.m→Class.m）」の注記を追記（ユーザー向け説明を簡潔に）。
 
+Index Operator Bring‑up（Phase‑20.31 内の小粒対応）
+
+目的
+- `expr[index]` の最小サポート（Array/Map の読み書き）。実行は NyRT dotted extern に正規化（get/set）。
+
+仕様（Phase‑1）
+- 読み取り: `arr[i]`, `map[k]`
+- 書き込み: `arr[i] = v`, `map[k] = v`
+- 文字列 index/range は後続（Phase‑2）
+- 未対応型は Fail‑Fast: "index operator is only supported for Array/Map"
+
+実装計画
+1) AST: IndexExpr と Assign(IndexExpr, …)（Rust パーサー）
+2) MIR Lowering: Array/Map の get/set に正規化
+3) スモーク（quick）: arr_read / arr_write / map_rw / negative_string
+4) ドキュメント: docs/specs/language/index-operator.md
+
+ロールアウト
+- 必要なら dev フラグ（HAKO_INDEX_OPERATOR_DEV=1）で段階導入（dev=ON, prod=OFF）。
+
+受け入れ基準
+- 上記スモークが PASS。未対応型は安定診断で Fail。
+
 Docs — Added
 - Unified method resolution design note: docs/development/builder/unified-method-resolution.md
   - Pipeline, invariants, flags, rollout plan（P4 observe → dev opt‑in → consider default）を整理。

docs/specs/language/index-operator.md

@@ -0,0 +1,41 @@
+Index Operator — Design (Phase‑20.31 scoped)
+
+Scope (Phase‑1)
+- Support bracket indexing for Array/Map:
+  - Read: expr[index]
+  - Write: expr[index] = value
+- Semantics (lowering):
+  - Array
+    - Read → nyash.array.get_h(handle, idx)
+    - Write → nyash.array.set_h(handle, idx, val)
+  - Map
+    - Read → nyash.map.get_hh(handle, key_any)
+    - Write → nyash.map.set_hh(handle, key_any, val_any)
+- Out of scope in Phase‑1: String indexing/ranges.
+
+Parser/AST
+- Add IndexExpr(target, index)
+- Permit Assign(IndexExpr, value) on the LHS.
+
+MIR Lowering (Fail‑Fast contract)
+- If receiver is neither Array nor Map → error: "index operator is only supported for Array/Map".
+- For Map keys/values:
+  - If arguments are SSA values (handles), pass as is; otherwise wrap primitive i64 as IntegerBox via available builder utilities.
+
+Error Policy
+- Fail‑Fast, no silent fallback.
+- Stable diagnostics for unsupported types and malformed LHS.
+
+Rollout / Flags
+- Implement in Rust parser first; Ny parser later.
+- If needed, guard with a dev flag for initial rollout (HAKO_INDEX_OPERATOR_DEV=1), default ON in dev profile, OFF in prod profiles.
+
+Tests (canaries)
+- arr_read: [1,2][0] == 1 → true
+- arr_write: arr[1]=3; arr[1]==3 → true
+- map_rw: m[10]=7; m[10]==7 → true
+- negative: "hello"[0] (Phase‑1) → error
+
+Notes
+- This leverages existing NyRT dotted externs already implemented for Array/Map, minimizing surface area and risk.
+