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 # Self Current Task — Now (main)
 
+## 2025-12-18：Phase 138 完了 ✅
+
+**Phase 138: ReturnValueLowererBox - Return Lowering SSOT**
+- 目的: Return lowering logic を共有 Box として抽出し SSOT を確立
+- 実装:
+  - Created `common/return_value_lowerer_box.rs` (~300 lines)
+  - Migrated `loop_true_break_once.rs` to use Box (2 call sites)
+  - 5 comprehensive unit tests for all patterns
+  - Code reduction: ~115 lines removed from loop_true_break_once.rs
+- SSOT:
+  - `ReturnValueLowererBox::lower_to_value_id()`
+  - Supported: Variable, Integer literal, Add expression (x + 2, 5 + 3)
+  - Fallback: Out-of-scope patterns return `Ok(None)`
+- Tests:
+  - 5 new unit tests: variable, integer literal, add (var+int), add (int+int), fallback (subtract)
+  - All regressions PASS: Phase 137/97/131/135/136
+- Design Decision:
+  - Phase 138 P0 migrated loop paths only
+  - Phase 139 P0 will unify post_if_post_k.rs for complete SSOT
+- Architecture Impact:
+  - Single location for return lowering improvements
+  - Isolated unit tests for return lowering logic
+  - Easy to add new return patterns in one location
+- 入口: `docs/development/current/main/phases/phase-138/README.md`
+
+---
+
 ## 2025-12-18：Phase 137 完了 ✅
 
 **Phase 137: loop(true) break-once with return add expression**
diff --git a/docs/development/current/main/phases/phase-138/README.md b/docs/development/current/main/phases/phase-138/README.md
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+# Phase 138: ReturnValueLowererBox - Return Lowering SSOT
+
+**Date**: 2025-12-18
+**Status**: DONE ✅
+**Scope**: Extract return lowering to shared Box (behavior unchanged)
+
+---
+
+## Goal
+
+Establish SSOT for return value lowering by extracting logic to ReturnValueLowererBox:
+- Extract `lower_return_value_to_vid()` from `loop_true_break_once.rs`
+- Create shared `ReturnValueLowererBox` for Normalized shadow paths
+- Keep **挙動不変**: Phase 136/137 behavior unchanged
+- Keep **Phase 139 準備**: Prepare for post_if_post_k.rs unification
+
+## Scope
+
+### ✅ In Scope (Phase 138 P0)
+
+- **Boxification**: Extract return lowering to `common/return_value_lowerer_box.rs`
+- **Migration**: `loop_true_break_once.rs` uses Box (2 call sites)
+- **Tests**: 5 unit tests for all supported patterns
+- **Regression**: All Phase 136/137 smokes unchanged
+
+### ❌ Out of Scope (Phase 139)
+
+- **post_if_post_k.rs**: Not modified (different responsibility)
+- Unification planned for Phase 139 P0
+
+## SSOT Location
+
+**File**: `src/mir/control_tree/normalized_shadow/common/return_value_lowerer_box.rs`
+
+**Function**: `ReturnValueLowererBox::lower_to_value_id()`
+
+### Supported Patterns (Phase 136-137)
+
+1. **Variable**: `return x` → env lookup → ValueId
+2. **Integer literal**: `return 7` → Const generation → ValueId
+3. **Add expression**: `return x + 2` or `return 5 + 3` → BinOp(Add, lhs, rhs) → ValueId
+
+### Out-of-Scope Patterns
+
+- Other operators: `return x - 2` → `Ok(None)` (fallback)
+- Variable + variable: `return x + y` → `Ok(None)` (fallback)
+- Nested expressions: `return (x + 2) + 3` → `Ok(None)` (fallback)
+
+## Implementation
+
+### New Files
+
+1. **`src/mir/control_tree/normalized_shadow/common/return_value_lowerer_box.rs`** (~300 lines)
+   - Main function: `lower_to_value_id()`
+   - Helper: `lower_binary_op()` for Add expressions
+   - 5 comprehensive unit tests
+
+2. **`src/mir/control_tree/normalized_shadow/common/mod.rs`**
+   - Module export for common utilities
+
+### Modified Files
+
+1. **`src/mir/control_tree/normalized_shadow/mod.rs`**
+   - Added `pub mod common;`
+
+2. **`src/mir/control_tree/normalized_shadow/loop_true_break_once.rs`**
+   - Removed `lower_return_value_to_vid()` method (~115 lines)
+   - Added import: `use super::common::return_value_lowerer_box::ReturnValueLowererBox;`
+   - Updated 2 call sites:
+     - Line 417: post_k return processing
+     - Line 481: k_exit return processing
+   - Updated SSOT documentation (lines 29-43)
+
+### Call Sites (Phase 138 P0)
+
+**File**: `loop_true_break_once.rs`
+
+1. **post_k return processing** (line 417):
+```rust
+match ReturnValueLowererBox::lower_to_value_id(
+    value_ast,
+    &mut post_k_func.body,
+    &mut next_value_id,
+    &env_post_k
+)? {
+    Some(vid) => { post_k_func.body.push(JoinInst::Ret { value: Some(vid) }); }
+    None => { return Ok(None); }
+}
+```
+
+2. **k_exit return processing** (line 481):
+```rust
+match ReturnValueLowererBox::lower_to_value_id(
+    value_ast,
+    &mut k_exit_func.body,
+    &mut next_value_id,
+    &env_k_exit
+)? {
+    Some(vid) => { k_exit_func.body.push(JoinInst::Ret { value: Some(vid) }); }
+    None => { return Ok(None); }
+}
+```
+
+## Unit Tests (5 tests)
+
+**File**: `src/mir/control_tree/normalized_shadow/common/return_value_lowerer_box.rs`
+
+1. `test_lower_variable` - Variable lookup in env
+2. `test_lower_integer_literal` - Const instruction generation
+3. `test_lower_add_var_plus_int` - `x + 2` pattern
+4. `test_lower_add_int_plus_int` - `5 + 3` constant folding
+5. `test_out_of_scope_subtract` - Fallback for unsupported operators
+
+## Verification
+
+```bash
+# Build
+cargo build --release -p nyash-rust --features llvm
+
+# Unit tests
+cargo test --lib
+
+# Phase 137 regression (6 tests)
+bash tools/smokes/v2/profiles/integration/apps/phase137_loop_true_break_once_return_add_vm.sh
+bash tools/smokes/v2/profiles/integration/apps/phase137_loop_true_break_once_return_add_const_vm.sh
+bash tools/smokes/v2/profiles/integration/apps/phase137_loop_true_break_once_post_return_add_vm.sh
+bash tools/smokes/v2/profiles/integration/apps/phase137_loop_true_break_once_return_add_llvm_exe.sh
+bash tools/smokes/v2/profiles/integration/apps/phase137_loop_true_break_once_return_add_const_llvm_exe.sh
+bash tools/smokes/v2/profiles/integration/apps/phase137_loop_true_break_once_post_return_add_llvm_exe.sh
+
+# Phase 97 regression (2 tests)
+bash tools/smokes/v2/profiles/integration/apps/phase97_next_non_ws_llvm_exe.sh
+bash tools/smokes/v2/profiles/integration/apps/phase97_json_loader_escape_llvm_exe.sh
+
+# Phase 131/135/136 regression
+bash tools/smokes/v2/profiles/integration/apps/phase131_loop_true_break_once_vm.sh
+bash tools/smokes/v2/profiles/integration/apps/phase135_loop_true_break_once_post_empty_return_vm.sh
+bash tools/smokes/v2/profiles/integration/apps/phase136_loop_true_break_once_return_literal_vm.sh
+```
+
+## Acceptance Criteria
+
+- ✅ cargo test --lib: 1194 tests PASS (+5 new unit tests)
+- ✅ Phase 137 regression: 6/6 PASS
+- ✅ Phase 97 regression: 2/2 PASS
+- ✅ Phase 131/135/136 regression: 3/3 PASS
+- ✅ Behavior unchanged: All Phase 136/137 fixtures/smokes PASS
+- ✅ SSOT established: ReturnValueLowererBox is single source of truth
+- ✅ post_if_post_k.rs unchanged: Phase 139 P0 scope
+
+## Boxification Trigger
+
+**Condition**: When 2+ files need identical return lowering logic
+
+**Result**: Phase 138 P0 achieved this trigger with loop_true_break_once.rs migration
+
+**Next Step**: Phase 139 P0 - Migrate post_if_post_k.rs to ReturnValueLowererBox
+
+## Current Status
+
+Phase 138 - DONE ✅ (2025-12-18)
+
+SSOT established: `common/return_value_lowerer_box.rs`
+
+Return lowering unified for loop paths, ready for if-with-post paths in Phase 139.
+
+## Architecture Impact
+
+- **Code Reduction**: ~115 lines removed from loop_true_break_once.rs
+- **Maintainability**: Single location for return lowering improvements
+- **Testability**: Isolated unit tests for return lowering logic
+- **Extensibility**: Easy to add new return patterns in one location