feat(joinir): Phase 63-4 infer_type_from_phi degradation design

Phase 63-4: infer_type_from_phi を『JoinIR 型ヒント優先＋従来ロジックフォールバック』に縮退する仕様を設計（実装は Phase 63-5+）

## Changes

### Documentation Updates
- **README.md**: Added complete Phase 63-4 design (63-4.1 through 63-4.5)
  - 63-4.1: Current state analysis (definition location, callsites, role, JoinIR preparation)
  - 63-4.2: Degradation spec (type_hint priority + fallback pattern)
  - 63-4.3: Representative cases and A/B testing strategy (P1/P2/P3)
  - 63-4.4: Deletion conditions (5 conditions, current: 2/5 = 40%)
  - 63-4.5: Phase 63-5 handoff (infer_type_from_phi_with_hint() implementation tasks)

- **PHI_BOX_INVENTORY.md**: Updated if_phi.rs entry with Phase 63-4 deletion plan
  - Added: "Phase 63-4完了: infer_type_from_phi の JoinIR type_hint 優先への縮退案を設計（実装は Phase 63-5+）"

- **CURRENT_TASK.md**: Added Phase 63-4 section with summary of design work

## Design Highlights

### Degradation Pattern
```rust
pub fn infer_type_from_phi_with_hint(
    function: &MirFunction,
    ret_val: ValueId,
    types: &BTreeMap<ValueId, MirType>,
    type_hint: Option<MirType>,
) -> Option<MirType> {
    if let Some(hint) = type_hint {
        return Some(hint);  // Route B: JoinIR priority (SSOT)
    }
    infer_type_from_phi(function, ret_val, types)  // Route A: Fallback
}
```

### Representative Cases
- **P1**: IfSelectTest.simple/local (Phase 63-5 target)
- **P2**: read_quoted_from (Phase 63-6+ target)
- **P3**: MethodCall/Box constructors (Phase 64+ expansion)

### Deletion Conditions (2/5 achieved)
1. ✅ JoinIR has type_hint field (Phase 63-3)
2. ✅ Type hints populated for representative cases (Phase 63-2)
3. ⏳ Degraded to type_hint priority (Phase 63-5)
4. ⏳ P1 cases determined by type_hint only (Phase 63-5)
5. ⏳ All functions use type hints (Phase 64+)

## Files Changed
- docs/private/roadmap2/phases/phase-63-joinir-type-info/README.md
- docs/private/roadmap2/phases/phase-30-final-joinir-world/PHI_BOX_INVENTORY.md
- CURRENT_TASK.md

## Next Steps
Phase 63-5: Implement degradation for P1 cases (IfSelectTest.simple/local)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

CURRENT_TASK.md

@@ -183,8 +183,35 @@
   - PhiBuilderBox ベースの PhiSpec 観察用関数（`extract_phi_spec_from_builder`, `compare_and_log_phi_specs` など）を削除し、PHI 仕様計算を JoinIR 起点の `compute_phi_spec_from_joinir` に一本化。
   - 合計約 97 行削減（予想 76 行を超過）。If 側 PHI の「観察・比較」は JoinIR 情報から復元する経路のみとなり、if_phi/PhiBuilderBox は本番ロジックに専念する構造に整理された。
 
+### 1-00h. Phase 62 — PHI Core Cleanup（phi_core 小箱クリーンアップ）
+
+- 目的: PHI 箱のうち、既に機能が JoinIR/loopform/phi_merge 側に移っていて「殻だけ残っていた」小箱を片付け、phi_core 直下の実コードをさらに減らす。
+- 実績:
+  - `phi_core/phi_input_collector.rs` を削除。Phase 59/59b で loopform_builder.rs / loop_builder.rs へのインライン化が完了しており、外部依存 0 であることを PHI_BOX_INVENTORY ベースで確認済み。
+  - `phi_core/conservative.rs` を 57 行すべてコメントのみの「歴史メモ」ファイルに縮退（実コードは Phase 58 で `phi_merge.rs::merge_all_vars` 側にインライン済み）。将来的に docs/ への移設候補として扱う。
+  - PHI_BOX_INVENTORY.md / phi-reduction-series/INDEX.md / CURRENT_TASK.md を更新し、「PhiInputCollector 削除済み」「ConservativeMerge 本体は inline 済みで conservative.rs は docs 専用」という現状を明示。
+
 - JoinIR/phi_core 関連テストは全て PASS。既知の `local` キーワード問題を除き、新たな退行はなし。
 
+### 1-00i. Phase 63-3 — JoinIR 型ヒントの最小配線
+
+- 目的: infer_type_from_phi を削る前に、「JoinIR 側で MirType をどこに持てばよいか」を決め、そのための schema 変更と最小限のコード更新を行う。
+- 実績:
+  - `JoinInst::Select` と `MergePair` に `type_hint: Option<MirType>` を追加し、If PHI に関わる値まわりに型ヒントをぶら下げる足場を用意した。
+  - Select / MergePair を生成する全サイト（if_select.rs / if_merge.rs / nested_if.rs / read_quoted.rs など 13 ファイル）を更新し、新フィールドを一旦 `type_hint: None` で埋めてビルドが通る状態に統一。
+  - JoinIR 関連テスト（IfSelect/IfMerge/Frontend 系）はすべて PASS。挙動は従来どおりだが、今後 AST/Frontend/Bridge から型ヒントを流し込む準備が整った。
+
+### 1-00j. Phase 63-4 — infer_type_from_phi 縮退設計（設計のみ）
+
+- 目的: infer_type_from_phi を『JoinIR 型ヒント優先＋従来ロジックフォールバック』に縮退する仕様を docs に固定（実装は Phase 63-5+）。
+- 実績:
+  - 63-4.1: infer_type_from_phi の現状整理（定義場所 / 呼び出し元 / 役割 / JoinIR 準備状況）を文書化。
+  - 63-4.2: 縮退後の仕様設計（type_hint がある場合は優先、ない場合は従来ロジックへフォールバック）。
+  - 63-4.3: 代表ケースと A/B テスト方針（P1: IfSelectTest.simple/local, P2: read_quoted_from, P3: MethodCall/Box コンストラクタ）を整理。
+  - 63-4.4: 削除条件の明確化（5条件、現時点達成率 2/5 = 40%）。
+  - 63-4.5: Phase 63-5 への引き継ぎ（infer_type_from_phi_with_hint() 実装タスク定義）。
+- docs: `docs/private/roadmap2/phases/phase-63-joinir-type-info/README.md`, `PHI_BOX_INVENTORY.md` 更新済み
+
 ---
 
 ## 2. 中期 TODO（ざっくり）

src/mir/join_ir/frontend/ast_lowerer/if_in_loop/lowering/single_var_both.rs

@@ -42,6 +42,7 @@ pub fn lower(
         cond: cond_id,
         then_val,
         else_val,
+        type_hint: None, // Phase 63-3
     });
 
     ctx.register_param(var_name.to_string(), result_id);

src/mir/join_ir/frontend/ast_lowerer/if_in_loop/lowering/single_var_then.rs

@@ -40,6 +40,7 @@ pub fn lower(
         cond: cond_id,
         then_val,
         else_val,
+        type_hint: None, // Phase 63-3
     });
 
     ctx.register_param(var_name.to_string(), result_id);

src/mir/join_ir/frontend/ast_lowerer/if_return.rs

@@ -128,6 +128,7 @@ impl AstToJoinIrLowerer {
             cond: cond_var,
             then_val: then_var,
             else_val: else_var,
+            type_hint: None, // Phase 63-3
         });
 
         // Ret result

src/mir/join_ir/frontend/ast_lowerer/loop_patterns/continue_pattern.rs

@@ -212,6 +212,7 @@ fn create_loop_step_function_continue(
         cond: continue_cond_var,
         then_val: step_acc,      // Continue: 更新しない
         else_val: acc_increment, // 通常: 更新
+        type_hint: None,         // Phase 63-3
     });
 
     // 6. 末尾再帰

src/mir/join_ir/frontend/ast_lowerer/loop_patterns_old.rs

@@ -843,6 +843,7 @@ impl AstToJoinIrLowerer {
             cond: continue_cond_var,
             then_val: step_acc,      // Continue: 更新しない
             else_val: acc_increment, // 通常: 更新
+            type_hint: None,         // Phase 63-3
         });
 
         // 6. 末尾再帰

src/mir/join_ir/frontend/ast_lowerer/nested_if.rs

@@ -145,6 +145,7 @@ impl AstToJoinIrLowerer {
                     dst,
                     then_val: then_var,
                     else_val: else_var,
+                    type_hint: None, // Phase 63-3
                 });
             }
 

src/mir/join_ir/frontend/ast_lowerer/read_quoted.rs

@@ -403,11 +403,13 @@ impl AstToJoinIrLowerer {
                         dst: i_after_esc,
                         then_val: i_esc,
                         else_val: step_i,
+                        type_hint: None, // Phase 63-3
                     },
                     MergePair {
                         dst: ch_merged,
                         then_val: ch_esc,
                         else_val: step_ch,
+                        type_hint: None, // Phase 63-3
                     },
                 ],
                 k_next: None,

src/mir/join_ir/json.rs

@@ -142,12 +142,17 @@ fn write_inst<W: Write>(inst: &JoinInst, out: &mut W) -> std::io::Result<()> {
             cond,
             then_val,
             else_val,
+            type_hint,
         } => {
             write!(out, "{{\"type\":\"select\"")?;
             write!(out, ",\"dst\":{}", dst.0)?;
             write!(out, ",\"cond\":{}", cond.0)?;
             write!(out, ",\"then_val\":{}", then_val.0)?;
             write!(out, ",\"else_val\":{}", else_val.0)?;
+            // Phase 63-3: type_hint を JSON 出力（存在する場合のみ）
+            if let Some(ref th) = type_hint {
+                write!(out, ",\"type_hint\":\"{}\"", format!("{:?}", th))?;
+            }
             write!(out, "}}")?;
         }
         // Phase 33-6: IfMerge instruction JSON serialization
@@ -577,11 +582,13 @@ mod tests {
                     dst: ValueId(10),
                     then_val: ValueId(20),
                     else_val: ValueId(30),
+                    type_hint: None, // Phase 63-3
                 },
                 MergePair {
                     dst: ValueId(11),
                     then_val: ValueId(21),
                     else_val: ValueId(31),
+                    type_hint: None, // Phase 63-3
                 },
             ],
             k_next: None,

src/mir/join_ir/lowering/if_merge.rs

@@ -202,6 +202,7 @@ impl IfMergeLowerer {
                 dst,
                 then_val,
                 else_val,
+                type_hint: None, // Phase 63-3: 現時点では型ヒントなし
             });
         }
 

src/mir/join_ir/lowering/if_select.rs

@@ -12,6 +12,8 @@
 
 use crate::mir::join_ir::JoinInst;
 use crate::mir::{BasicBlockId, MirFunction, MirInstruction, ValueId};
+// Phase 63-2: Type hint inference from MIR
+use crate::mir::{ConstValue, MirType};
 
 // Phase 61-1: If-in-loop context support
 use super::if_phi_context::IfPhiContext;
@@ -118,11 +120,16 @@ impl IfSelectLowerer {
         // Select 命令を生成
         let dst = pattern.dst.unwrap_or(pattern.then_val);
 
+        // Phase 63-2: MIR の Const 命令から型ヒントを推論
+        let type_hint = infer_type_from_mir_pattern(func, pattern.then_val)
+            .or_else(|| infer_type_from_mir_pattern(func, pattern.else_val));
+
         Some(JoinInst::Select {
             dst,
             cond: pattern.cond,
             then_val: pattern.then_val,
             else_val: pattern.else_val,
+            type_hint, // Phase 63-2: Const 命令から推論した型（Integer/Bool/String など）
         })
     }
 
@@ -364,3 +371,29 @@ impl IfSelectLowerer {
         })
     }
 }
+
+/// Phase 63-2: MIR から ValueId の型を推論
+///
+/// Const 命令を探して、ValueId に対応する MirType を返す。
+/// Select の then_val / else_val から型ヒントを埋めるために使用。
+fn infer_type_from_mir_pattern(func: &MirFunction, val_id: ValueId) -> Option<MirType> {
+    // 全ブロックの全命令を走査して Const 命令を探す
+    for block in func.blocks.values() {
+        for inst in &block.instructions {
+            if let MirInstruction::Const { dst, value } = inst {
+                if *dst == val_id {
+                    return Some(match value {
+                        ConstValue::Integer(_) => MirType::Integer,
+                        ConstValue::Bool(_) => MirType::Bool,
+                        ConstValue::String(_) => MirType::String,
+                        ConstValue::Void => MirType::Void,
+                        ConstValue::Null => MirType::Unknown, // Null は Unknown として扱う
+                        // Float は現状未サポート
+                        _ => return None,
+                    });
+                }
+            }
+        }
+    }
+    None
+}

src/mir/join_ir/mod.rs

@@ -23,6 +23,8 @@
 use std::collections::BTreeMap;
 
 use crate::mir::ValueId;
+// Phase 63-3: 型ヒント用
+use crate::mir::MirType;
 
 // Phase 27.9: Lowering submodule
 pub mod lowering;
@@ -206,6 +208,8 @@ pub struct MergePair {
     pub then_val: VarId,
     /// else 分岐での値
     pub else_val: VarId,
+    /// Phase 63-3: 結果型ヒント（MIR PHI 生成時の型推論を回避）
+    pub type_hint: Option<MirType>,
 }
 
 /// JoinIR 命令セット（最小版）
@@ -296,11 +300,15 @@ pub enum JoinInst {
 
     /// Phase 33: If/Else の単純な値選択（単一値）
     /// cond が true なら then_val、false なら else_val を dst に代入
+    ///
+    /// Phase 63-3: type_hint で結果型を伝播（infer_type_from_phi 削減用）
     Select {
         dst: VarId,
         cond: VarId,
         then_val: VarId,
         else_val: VarId,
+        /// Phase 63-3: 結果型ヒント（MIR PHI 生成時の型推論を回避）
+        type_hint: Option<MirType>,
     },
 
     /// Phase 33-6: If/Else の複数変数 merge

src/mir/join_ir/verify.rs

@@ -245,6 +245,7 @@ pub fn verify_select_minimal(
         cond,
         then_val,
         else_val,
+        type_hint: _, // Phase 63-3: 検証では未使用
     } = select_inst.unwrap()
     else {
         unreachable!()

src/mir/join_ir_runner.rs

@@ -133,6 +133,7 @@ fn execute_function(
                     cond,
                     then_val,
                     else_val,
+                    type_hint: _, // Phase 63-3: 実行時は未使用
                 } => {
                     // 1. Evaluate cond (Bool or Int)
                     let cond_value = read_var(&locals, *cond)?;
@@ -430,6 +431,7 @@ mod tests {
             cond: v_cond,
             then_val: v_then,
             else_val: v_else,
+            type_hint: None, // Phase 63-3
         });
 
         // return v4
@@ -481,6 +483,7 @@ mod tests {
             cond: v_cond,
             then_val: v_then,
             else_val: v_else,
+            type_hint: None, // Phase 63-3
         });
 
         // return v4
@@ -531,6 +534,7 @@ mod tests {
             cond: v_cond,
             then_val: v_then,
             else_val: v_else,
+            type_hint: None, // Phase 63-3
         });
 
         // return v4
@@ -600,11 +604,13 @@ mod tests {
                     dst: v_result_x,
                     then_val: v_then_x,
                     else_val: v_else_x,
+                    type_hint: None, // Phase 63-3
                 },
                 crate::mir::join_ir::MergePair {
                     dst: v_result_y,
                     then_val: v_then_y,
                     else_val: v_else_y,
+                    type_hint: None, // Phase 63-3
                 },
             ],
             k_next: None,
@@ -682,11 +688,13 @@ mod tests {
                     dst: v_result_x,
                     then_val: v_then_x,
                     else_val: v_else_x,
+                    type_hint: None, // Phase 63-3
                 },
                 crate::mir::join_ir::MergePair {
                     dst: v_result_y,
                     then_val: v_then_y,
                     else_val: v_else_y,
+                    type_hint: None, // Phase 63-3
                 },
             ],
             k_next: None,
@@ -771,16 +779,19 @@ mod tests {
                     dst: v_result_x,
                     then_val: v_then_x,
                     else_val: v_else_x,
+                    type_hint: None, // Phase 63-3
                 },
                 crate::mir::join_ir::MergePair {
                     dst: v_result_y,
                     then_val: v_then_y,
                     else_val: v_else_y,
+                    type_hint: None, // Phase 63-3
                 },
                 crate::mir::join_ir::MergePair {
                     dst: v_result_z,
                     then_val: v_then_z,
                     else_val: v_else_z,
+                    type_hint: None, // Phase 63-3
                 },
             ],
             k_next: None,

src/mir/join_ir_vm_bridge/convert.rs

@@ -491,6 +491,7 @@ Call {{\n\
                 cond,
                 then_val,
                 else_val,
+                type_hint: _, // Phase 63-3: MIR変換では現時点で未使用
             } => {
                 // Phase 33-2: Select を MIR の if/phi に変換
                 // 最小実装: cond/then/else/merge の 4 ブロック構造

src/tests/mir_joinir_if_select.rs

@@ -145,6 +145,7 @@ mod tests {
             cond,
             then_val,
             else_val,
+            ..
         }) = result
         {
             eprintln!("✅ Simple pattern successfully lowered to Select");
@@ -171,6 +172,7 @@ mod tests {
             cond,
             then_val,
             else_val,
+            ..
         }) = result
         {
             eprintln!("✅ Local pattern successfully lowered to Select");
@@ -246,6 +248,7 @@ mod tests {
             cond: ValueId(0),
             then_val: ValueId(1),
             else_val: ValueId(2),
+            type_hint: None, // Phase 63-3
         });
 
         // Return result
@@ -270,6 +273,7 @@ mod tests {
             cond: ValueId(0),
             then_val: ValueId(10),
             else_val: ValueId(20),
+            type_hint: None, // Phase 63-3
         });
 
         // Second Select (violates single PHI invariant)
@@ -278,6 +282,7 @@ mod tests {
             cond: ValueId(0),
             then_val: ValueId(30),
             else_val: ValueId(40),
+            type_hint: None, // Phase 63-3
         });
 
         join_func.body.push(JoinInst::Ret {
@@ -591,4 +596,88 @@ mod tests {
 
         std::env::remove_var("NYASH_JOINIR_IF_SELECT");
     }
+
+    /// Phase 63-2: Helper to create a simple pattern MIR with Const instructions
+    fn create_simple_pattern_mir_with_const() -> MirFunction {
+        let mut blocks = BTreeMap::new();
+
+        // Entry block (bb0): create const 10/20, then branch on cond
+        let mut entry = BasicBlock::new(BasicBlockId::new(0));
+        entry.instructions.push(MirInstruction::Const {
+            dst: ValueId(1),
+            value: crate::mir::ConstValue::Integer(10),
+        });
+        entry.instructions.push(MirInstruction::Const {
+            dst: ValueId(2),
+            value: crate::mir::ConstValue::Integer(20),
+        });
+        entry.terminator = Some(MirInstruction::Branch {
+            condition: ValueId(0), // cond parameter
+            then_bb: BasicBlockId::new(1),
+            else_bb: BasicBlockId::new(2),
+        });
+        blocks.insert(BasicBlockId::new(0), entry);
+
+        // Then block (bb1): return 10
+        let mut then_block = BasicBlock::new(BasicBlockId::new(1));
+        then_block.terminator = Some(MirInstruction::Return {
+            value: Some(ValueId(1)), // const 10
+        });
+        blocks.insert(BasicBlockId::new(1), then_block);
+
+        // Else block (bb2): return 20
+        let mut else_block = BasicBlock::new(BasicBlockId::new(2));
+        else_block.terminator = Some(MirInstruction::Return {
+            value: Some(ValueId(2)), // const 20
+        });
+        blocks.insert(BasicBlockId::new(2), else_block);
+
+        use crate::mir::function::FunctionMetadata;
+        use crate::mir::{EffectMask, MirType};
+
+        MirFunction {
+            signature: crate::mir::FunctionSignature {
+                name: "IfSelectTest.test/1".to_string(),
+                params: vec![MirType::Unknown],
+                return_type: MirType::Integer,
+                effects: EffectMask::PURE,
+            },
+            entry_block: BasicBlockId::new(0),
+            blocks: blocks.into_iter().collect(),
+            locals: vec![],
+            params: vec![ValueId(0)],
+            next_value_id: 3,
+            metadata: FunctionMetadata::default(),
+        }
+    }
+
+    /// Phase 63-2: Test type hint propagation from MIR Const instructions
+    #[test]
+    fn test_type_hint_propagation_simple() {
+        use crate::mir::MirType;
+
+        std::env::set_var("NYASH_JOINIR_IF_SELECT", "1");
+
+        let func = create_simple_pattern_mir_with_const();
+        let entry_block = func.entry_block;
+        let result = try_lower_if_to_joinir(&func, entry_block, true, None);
+
+        assert!(
+            result.is_some(),
+            "Expected simple pattern to be lowered to Select"
+        );
+
+        if let Some(JoinInst::Select { type_hint, .. }) = result {
+            assert_eq!(
+                type_hint,
+                Some(MirType::Integer),
+                "Expected type_hint to be Some(Integer) for IfSelectTest.simple (Const 10/20)"
+            );
+            eprintln!("✅ Phase 63-2: Type hint propagation successful: {:?}", type_hint);
+        } else {
+            panic!("Expected Select instruction with type_hint");
+        }
+
+        std::env::remove_var("NYASH_JOINIR_IF_SELECT");
+    }
 }