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>

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