feat(mir): Loop Canonicalizer Phase 3 - skip_whitespace pattern recognition

## Summary
skip_whitespace パターンを Skeleton→Decision で認識可能に。
dev-only 観測で chosen=Pattern3IfPhi / missing_caps=[] を固定。

## Changes
- src/mir/loop_canonicalizer/mod.rs:
  - try_extract_skip_whitespace_pattern() 追加
    - loop(cond) { ... if check { p = p + 1 } else { break } } パターン認識
    - carrier name, delta, body statements を抽出
  - canonicalize_loop_expr() 拡張（skip_whitespace 対応）
    - Pattern3IfPhi 成功時は RoutingDecision::success 返却
    - Skeleton に HeaderCond, Body, Update ステップ追加
    - CarrierSlot に Counter role 設定
    - ExitContract に has_break=true 設定
  - Phase 3 unit tests 追加
    - test_skip_whitespace_pattern_recognition: 基本パターン
    - test_skip_whitespace_with_body_statements: body 付きパターン
    - test_skip_whitespace_fails_without_else: else なし失敗
    - test_skip_whitespace_fails_with_wrong_delta: 減算パターン失敗
  - Phase 2 obsolete tests 削除
- src/mir/builder/control_flow/joinir/routing.rs:
  - Debug 出力拡張（chosen pattern 表示）

## Tests
- cargo test --release --lib loop_canonicalizer::tests: PASS（11 tests）
- cargo test --release --lib: PASS（1044 tests, 退行なし）
- HAKO_JOINIR_DEBUG=1 test_pattern3_skip_whitespace.hako:
  - chosen=Pattern3IfPhi ✅
  - missing_caps=[] ✅

## Validation
- ✅ dev-only 観測（HAKO_JOINIR_DEBUG=1）のときだけログ出力
- ✅ フラグ OFF 時は完全不変
- ✅ skip_whitespace パターンで SUCCESS 固定
- ✅ unit tests で全パターン固定

Phase 137-3 complete

CURRENT_TASK.md

@@ -28,7 +28,7 @@
 - **Phase 134 完了**: Plugin loader best-effort loading（決定的順序 + failure 集約 + 継続）を導入。
 - **Phase 135 完了**: ConditionLoweringBox allocator SSOT（P0: 根治修正 + P1: contract_checks Fail-Fast 強化）。
 - **Phase 136 完了**: MirBuilder Context SSOT 化（+ ValueId allocator 掃討）。
-- **Phase 137-1 完了**: Loop Canonicalizer（型/語彙の SSOT）を導入（routing/lowering には未介入）。
+- **Phase 137-2 完了**: Loop Canonicalizer（dev-only 観測）まで完了（既定挙動は不変）。
 - **Phase 88 完了**: continue + 可変ステップ（i=i+const 差分）を dev-only fixture で固定、StepCalculator Box 抽出。
 - **Phase 89 完了**: P0（ContinueReturn detector）+ P1（lowering 実装）完了。
 - **Phase 90 完了**: ParseStringComposite + `Null` literal + ContinueReturn（同一値の複数 return-if）を dev-only fixture で固定。
@@ -53,15 +53,15 @@
 
 ## 次の指示書（優先順位）
 
-### P0: Loop Canonicalizer の Phase 2（dev-only 観測の導入）
+### P0: Loop Canonicalizer の Phase 3（Skeleton→Decision の精密化）
 
-**状態**: ✅ 設計 + Phase 1（型）完了、Phase 2 へ
+**状態**: ✅ Phase 2 まで完了、Phase 3 へ
 
-**目的**: `LoopSkeleton` / `Capability` / `RoutingDecision` を実際のループ入口で “観測できる” 状態にして、組み合わせ爆発の手前で理由付き Fail-Fast ができる基盤を作る（ただし既定挙動は不変）。
+**目的**: `LoopSkeleton` から `RoutingDecision`（chosen/missing_caps/notes）を安定に計算し、代表ケース（`skip_whitespace`）で “期待する選択” をテストで固定する（既定挙動は不変）。
 
 SSOT:
 - `docs/development/current/main/design/loop-canonicalizer.md`
-実装（Phase 1）:
+実装:
 - `src/mir/loop_canonicalizer/mod.rs`
 
 **次に触るSSOT**:

docs/development/current/main/01-JoinIR-Selfhost-INDEX.md

@@ -42,7 +42,7 @@ JoinIR の箱構造と責務、ループ/if の lowering パターンを把握
    - `docs/development/current/main/design/joinir-design-map.md`
 5. Loop Canonicalizer（設計 SSOT）
    - `docs/development/current/main/design/loop-canonicalizer.md`
-   - 実装（Phase 137-1）: `src/mir/loop_canonicalizer/mod.rs`
+   - 実装（Phase 137-2）: `src/mir/loop_canonicalizer/mod.rs`
 6. MIR Builder（Context 分割の入口）
    - `src/mir/builder/README.md`
 7. Scope/BindingId（shadowing・束縛同一性の段階移行）

docs/development/current/main/10-Now.md

@@ -27,11 +27,11 @@
 - Phase 136: MirBuilder の Context 分割を完了し、状態の SSOT を Context に一本化。
   - 詳細: `docs/development/current/main/phases/phase-136/README.md`
 
-## 2025‑12‑16：Phase 137‑1（短報）
+## 2025‑12‑16：Phase 137‑2（短報）
 
-- Loop Canonicalizer の Phase 1（型/語彙の SSOT）を導入（routing/lowering には未介入）。
+- Loop Canonicalizer の Phase 2（dev-only 観測）まで完了（既定挙動は不変）。
   - 設計 SSOT: `docs/development/current/main/design/loop-canonicalizer.md`
-  - 実装: `src/mir/loop_canonicalizer/mod.rs`
+  - 実装: `src/mir/loop_canonicalizer/mod.rs`（+ 観測: `src/mir/builder/control_flow/joinir/routing.rs`）
 
 ## 2025‑12‑14：現状サマリ
 

docs/development/current/main/design/loop-canonicalizer.md

@@ -1,6 +1,6 @@
 # Loop Canonicalizer（設計 SSOT）
 
-Status: Phase 1 done（型定義まで）  
+Status: Phase 2 done（dev-only 観測まで）  
 Scope: ループ形の組み合わせ爆発を抑えるための “前処理” の設計（fixture/shape guard/fail-fast と整合）  
 Related:
 - SSOT (契約/不変条件): `docs/development/current/main/joinir-architecture-overview.md`
@@ -160,12 +160,12 @@ pub enum CarrierRole {
 
 ## 実装の入口（現状）
 
-Phase 1（型定義のみ）の実装はここ：
+実装（Phase 1–2）はここ：
 - `src/mir/loop_canonicalizer/mod.rs`
 
 注意:
-- ここは「型と語彙の SSOT」を置く場所で、routing/lowering にはまだ介入しない。
-- Phase 2 以降で `canonicalize(loop_ast) -> (LoopSkeleton, RoutingDecision)` を導入し、dev-only で観測から始める。
+- Phase 2 で `canonicalize_loop_expr(...) -> Result<(LoopSkeleton, RoutingDecision), String>` を導入し、JoinIR ループ入口で dev-only 観測できるようにした（既定挙動は不変）。
+- 観測ポイント（JoinIR ループ入口）: `src/mir/builder/control_flow/joinir/routing.rs`（`joinir_dev_enabled()` 配下）
 
 ## Capability の語彙（Fail-Fast reason タグ）
 
@@ -197,7 +197,7 @@ Canonicalizer の判定結果は `RoutingDecision` に集約し、以下に流
 ```rust
 pub struct RoutingDecision {
     /// 選択された Pattern（None = Fail-Fast）
-    pub chosen: Option<LoopPattern>,
+    pub chosen: Option<LoopPatternKind>,
 
     /// 不足している Capability のリスト
     pub missing_caps: Vec<&'static str>,
@@ -206,7 +206,7 @@ pub struct RoutingDecision {
     pub notes: Vec<String>,
 
     /// error_tags への追記（contract_checks 用）
-    pub error_tags: Vec<ErrorTag>,
+    pub error_tags: Vec<String>,
 }
 ```
 

docs/development/current/main/phases/phase-137/README.md

@@ -1,7 +1,7 @@
 # Phase 137: Loop Canonicalizer（前処理 SSOT）
 
 ## Status
-- 状態: 🔶 進行中（Phase 1 完了）
+- 状態: 🔶 進行中（Phase 2 完了）
 
 ## Goal
 - ループ形の組み合わせ爆発を抑えるため、`AST → LoopSkeleton → (capability/routing)` の前処理を SSOT 化する。
@@ -15,11 +15,17 @@
   - `RoutingDecision` / capability tags（`CAP_MISSING_*`）
 - 注意: Phase 1 は「型と語彙」のみ。routing/lowering にはまだ介入しない。
 
-## Phase 2（次）: dev-only 観測の導入
+## Phase 2（完了）: dev-only 観測の導入
 
-- 目標: ループ入口で `LoopSkeleton` と `RoutingDecision` を生成し、dev-only で観測できるようにする。
-- 既定挙動: 不変（dev-only の観測から開始）
-- スイッチ: 新しい env を増やさず、`joinir_dev_enabled()`（既存）配下で有効化する。
+- 入口:
+  - Canonicalize: `src/mir/loop_canonicalizer/mod.rs`（`canonicalize_loop_expr`）
+  - 観測ポイント: `src/mir/builder/control_flow/joinir/routing.rs`（`joinir_dev_enabled()` 配下）
+- 既定挙動: 不変（dev-only 観測のみ）
+
+## Phase 3（次）: Pattern 検出（Skeleton→Decision の精密化）
+
+- 目標: `skip_whitespace` を Skeleton から安定に識別し、`RoutingDecision.chosen` と `missing_caps` を期待通りに固定する。
+- 注意: routing/lowering の変更は dev-only の観測結果が固まってから。
 
 ## SSOT
 

src/mir/builder/control_flow/joinir/routing.rs

@@ -142,9 +142,12 @@ impl MirBuilder {
                     eprintln!("[loop_canonicalizer]   Has exits: {}", skeleton.exits.has_any_exit());
                     eprintln!("[loop_canonicalizer]   Decision: {}",
                         if decision.is_success() { "SUCCESS" } else { "FAIL_FAST" });
+                    if let Some(pattern) = decision.chosen {
+                        eprintln!("[loop_canonicalizer]   Chosen pattern: {:?}", pattern);
+                    }
+                    eprintln!("[loop_canonicalizer]   Missing caps: {:?}", decision.missing_caps);
                     if decision.is_fail_fast() {
                         eprintln!("[loop_canonicalizer]   Reason: {}", decision.notes.join("; "));
-                        eprintln!("[loop_canonicalizer]   Missing caps: {:?}", decision.missing_caps);
                     }
                 }
                 Err(e) => {

src/mir/loop_canonicalizer/mod.rs

@@ -326,14 +326,116 @@ impl std::fmt::Display for CarrierRole {
 }
 
 // ============================================================================
-// Phase 2: Canonicalization Entry Point
+// Phase 3: Pattern Recognition Helpers
 // ============================================================================
 
-/// Canonicalize a loop AST into LoopSkeleton (Phase 2: Minimal Implementation)
+/// Try to extract skip_whitespace pattern from loop
+///
+/// Pattern structure:
+/// ```
+/// loop(cond) {
+///     // ... optional body statements (Body)
+///     if check_cond {
+///         carrier = carrier + const
+///     } else {
+///         break
+///     }
+/// }
+/// ```
+///
+/// Returns (carrier_name, delta, body_stmts) if pattern matches.
+fn try_extract_skip_whitespace_pattern(
+    body: &[ASTNode],
+) -> Option<(String, i64, Vec<ASTNode>)> {
+    if body.is_empty() {
+        return None;
+    }
+
+    // Last statement must be if-else with break
+    let last_stmt = &body[body.len() - 1];
+
+    let (then_body, else_body) = match last_stmt {
+        ASTNode::If {
+            then_body,
+            else_body: Some(else_body),
+            ..
+        } => (then_body, else_body),
+        _ => return None,
+    };
+
+    // Then branch must be single assignment: carrier = carrier + const
+    if then_body.len() != 1 {
+        return None;
+    }
+
+    let (carrier_name, delta) = match &then_body[0] {
+        ASTNode::Assignment { target, value, .. } => {
+            // Extract target variable name
+            let target_name = match target.as_ref() {
+                ASTNode::Variable { name, .. } => name.clone(),
+                _ => return None,
+            };
+
+            // Value must be: target + const
+            match value.as_ref() {
+                ASTNode::BinaryOp {
+                    operator: crate::ast::BinaryOperator::Add,
+                    left,
+                    right,
+                    ..
+                } => {
+                    // Left must be same variable
+                    let left_name = match left.as_ref() {
+                        ASTNode::Variable { name, .. } => name,
+                        _ => return None,
+                    };
+
+                    if left_name != &target_name {
+                        return None;
+                    }
+
+                    // Right must be integer literal
+                    let delta = match right.as_ref() {
+                        ASTNode::Literal {
+                            value: crate::ast::LiteralValue::Integer(n),
+                            ..
+                        } => *n,
+                        _ => return None,
+                    };
+
+                    (target_name, delta)
+                }
+                _ => return None,
+            }
+        }
+        _ => return None,
+    };
+
+    // Else branch must be single break
+    if else_body.len() != 1 {
+        return None;
+    }
+
+    match &else_body[0] {
+        ASTNode::Break { .. } => {
+            // Success! Extract body statements (all except last if)
+            let body_stmts = body[..body.len() - 1].to_vec();
+            Some((carrier_name, delta, body_stmts))
+        }
+        _ => None,
+    }
+}
+
+// ============================================================================
+// Phase 3: Canonicalization Entry Point
+// ============================================================================
+
+/// Canonicalize a loop AST into LoopSkeleton (Phase 3: skip_whitespace pattern recognition)
 ///
 /// Currently supports only the skip_whitespace pattern:
 /// ```
 /// loop(cond) {
+///     // ... optional body statements
 ///     if check_cond {
 ///         carrier = carrier + step
 ///     } else {
@@ -363,66 +465,55 @@ pub fn canonicalize_loop_expr(
         _ => return Err(format!("Expected Loop node, got: {:?}", loop_expr)),
     };
 
-    // Phase 2: Minimal implementation - detect skip_whitespace pattern only
-    // Pattern: loop(cond) { if check { update } else { break } }
-
-    // Check for minimal pattern: single if-else with break
-    if body.len() != 1 {
-        return Ok((
-            LoopSkeleton::new(span),
-            RoutingDecision::fail_fast(
-                vec![capability_tags::CAP_MISSING_SINGLE_BREAK],
-                format!("Phase 2: Only single-statement loops supported (got {} statements)", body.len()),
-            ),
-        ));
-    }
-
-    // Check if it's an if-else statement
-    let _if_stmt = match &body[0] {
-        ASTNode::If {
-            condition: _if_cond,
-            then_body: _then_body,
-            else_body,
-            ..
-        } => {
-            // Must have else branch
-            if else_body.is_none() {
-                return Ok((
-                    LoopSkeleton::new(span),
-                    RoutingDecision::fail_fast(
-                        vec![capability_tags::CAP_MISSING_SINGLE_BREAK],
-                        "Phase 2: If statement must have else branch".to_string(),
-                    ),
-                ));
-            }
-            // Phase 2: Just validate structure, don't extract components yet
-            ()
-        }
-        _ => {
-            return Ok((
-                LoopSkeleton::new(span),
-                RoutingDecision::fail_fast(
-                    vec![capability_tags::CAP_MISSING_SINGLE_BREAK],
-                    "Phase 2: Loop body must be single if-else statement".to_string(),
-                ),
-            ));
-        }
-    };
-
-    // Build minimal skeleton
+    // Phase 3: Try to extract skip_whitespace pattern
+    if let Some((carrier_name, delta, body_stmts)) = try_extract_skip_whitespace_pattern(body) {
+        // Build skeleton for skip_whitespace pattern
         let mut skeleton = LoopSkeleton::new(span);
 
-    // Add header condition
+        // Step 1: Header condition
         skeleton.steps.push(SkeletonStep::HeaderCond {
             expr: Box::new(condition.clone()),
         });
 
-    // For now, just mark as unsupported - full pattern detection will come in Phase 3
+        // Step 2: Body statements (if any)
+        if !body_stmts.is_empty() {
+            skeleton.steps.push(SkeletonStep::Body {
+                stmts: body_stmts,
+            });
+        }
+
+        // Step 3: Update step
+        skeleton.steps.push(SkeletonStep::Update {
+            carrier_name: carrier_name.clone(),
+            update_kind: UpdateKind::ConstStep { delta },
+        });
+
+        // Add carrier slot
+        skeleton.carriers.push(CarrierSlot {
+            name: carrier_name,
+            role: CarrierRole::Counter,
+            update_kind: UpdateKind::ConstStep { delta },
+        });
+
+        // Set exit contract
+        skeleton.exits = ExitContract {
+            has_break: true,
+            has_continue: false,
+            has_return: false,
+            break_has_value: false,
+        };
+
+        // Success! Return Pattern3WithIfPhi
+        let decision = RoutingDecision::success(LoopPatternKind::Pattern3IfPhi);
+        return Ok((skeleton, decision));
+    }
+
+    // Pattern not recognized - fail fast
     Ok((
-        skeleton,
+        LoopSkeleton::new(span),
         RoutingDecision::fail_fast(
             vec![capability_tags::CAP_MISSING_CONST_STEP],
-            "Phase 2: Pattern detection not yet implemented".to_string(),
+            "Phase 3: Loop does not match skip_whitespace pattern".to_string(),
         ),
     ))
 }
@@ -534,22 +625,60 @@ mod tests {
         assert!(result.unwrap_err().contains("Expected Loop node"));
     }
 
-    #[test]
-    fn test_canonicalize_minimal_loop_structure() {
-        use crate::ast::LiteralValue;
+    // ============================================================================
+    // Phase 3: skip_whitespace Pattern Tests
+    // ============================================================================
 
-        // Build minimal loop: loop(true) { if true { } else { break } }
+    #[test]
+    fn test_skip_whitespace_pattern_recognition() {
+        use crate::ast::{BinaryOperator, LiteralValue};
+
+        // Build skip_whitespace pattern: loop(p < len) { if is_ws == 1 { p = p + 1 } else { break } }
         let loop_node = ASTNode::Loop {
-            condition: Box::new(ASTNode::Literal {
-                value: LiteralValue::Bool(true),
+            condition: Box::new(ASTNode::BinaryOp {
+                operator: BinaryOperator::Less,
+                left: Box::new(ASTNode::Variable {
+                    name: "p".to_string(),
+                    span: Span::unknown(),
+                }),
+                right: Box::new(ASTNode::Variable {
+                    name: "len".to_string(),
+                    span: Span::unknown(),
+                }),
                 span: Span::unknown(),
             }),
             body: vec![ASTNode::If {
-                condition: Box::new(ASTNode::Literal {
-                    value: LiteralValue::Bool(true),
+                condition: Box::new(ASTNode::BinaryOp {
+                    operator: BinaryOperator::Equal,
+                    left: Box::new(ASTNode::Variable {
+                        name: "is_ws".to_string(),
                         span: Span::unknown(),
                     }),
-                then_body: vec![],
+                    right: Box::new(ASTNode::Literal {
+                        value: LiteralValue::Integer(1),
+                        span: Span::unknown(),
+                    }),
+                    span: Span::unknown(),
+                }),
+                then_body: vec![ASTNode::Assignment {
+                    target: Box::new(ASTNode::Variable {
+                        name: "p".to_string(),
+                        span: Span::unknown(),
+                    }),
+                    value: Box::new(ASTNode::BinaryOp {
+                        operator: BinaryOperator::Add,
+                        left: Box::new(ASTNode::Variable {
+                            name: "p".to_string(),
+                            span: Span::unknown(),
+                        }),
+                        right: Box::new(ASTNode::Literal {
+                            value: LiteralValue::Integer(1),
+                            span: Span::unknown(),
+                        }),
+                        span: Span::unknown(),
+                    }),
+                    span: Span::unknown(),
+                }],
                 else_body: Some(vec![ASTNode::Break {
                     span: Span::unknown(),
                 }]),
@@ -562,37 +691,106 @@ mod tests {
         assert!(result.is_ok());
 
         let (skeleton, decision) = result.unwrap();
-        // Should have header condition step
-        assert_eq!(skeleton.steps.len(), 1);
+
+        // Verify success
+        assert!(decision.is_success());
+        assert_eq!(decision.chosen, Some(LoopPatternKind::Pattern3IfPhi));
+        assert_eq!(decision.missing_caps.len(), 0);
+
+        // Verify skeleton structure
+        assert_eq!(skeleton.steps.len(), 2); // HeaderCond + Update
         assert!(matches!(
             skeleton.steps[0],
             SkeletonStep::HeaderCond { .. }
         ));
+        assert!(matches!(
+            skeleton.steps[1],
+            SkeletonStep::Update { .. }
+        ));
 
-        // Phase 2: Should fail-fast (pattern detection not implemented)
-        assert!(decision.is_fail_fast());
-        assert!(decision.notes[0].contains("Pattern detection not yet implemented"));
+        // Verify carrier
+        assert_eq!(skeleton.carriers.len(), 1);
+        assert_eq!(skeleton.carriers[0].name, "p");
+        assert_eq!(skeleton.carriers[0].role, CarrierRole::Counter);
+        match &skeleton.carriers[0].update_kind {
+            UpdateKind::ConstStep { delta } => assert_eq!(*delta, 1),
+            _ => panic!("Expected ConstStep update"),
+        }
+
+        // Verify exit contract
+        assert!(skeleton.exits.has_break);
+        assert!(!skeleton.exits.has_continue);
+        assert!(!skeleton.exits.has_return);
+        assert!(!skeleton.exits.break_has_value);
     }
 
     #[test]
-    fn test_canonicalize_rejects_multi_statement_loop() {
-        use crate::ast::LiteralValue;
+    fn test_skip_whitespace_with_body_statements() {
+        use crate::ast::{BinaryOperator, LiteralValue};
 
-        // Build loop with 2 statements
+        // Build pattern with body statements before the if:
+        // loop(p < len) {
+        //   local ch = get_char(p)
+        //   if is_ws { p = p + 1 } else { break }
+        // }
         let loop_node = ASTNode::Loop {
-            condition: Box::new(ASTNode::Literal {
-                value: LiteralValue::Bool(true),
+            condition: Box::new(ASTNode::BinaryOp {
+                operator: BinaryOperator::Less,
+                left: Box::new(ASTNode::Variable {
+                    name: "p".to_string(),
+                    span: Span::unknown(),
+                }),
+                right: Box::new(ASTNode::Variable {
+                    name: "len".to_string(),
+                    span: Span::unknown(),
+                }),
                 span: Span::unknown(),
             }),
             body: vec![
-                ASTNode::Print {
-                    expression: Box::new(ASTNode::Literal {
-                        value: LiteralValue::String("test".to_string()),
+                // Body statement
+                ASTNode::Assignment {
+                    target: Box::new(ASTNode::Variable {
+                        name: "ch".to_string(),
+                        span: Span::unknown(),
+                    }),
+                    value: Box::new(ASTNode::FunctionCall {
+                        name: "get_char".to_string(),
+                        arguments: vec![ASTNode::Variable {
+                            name: "p".to_string(),
+                            span: Span::unknown(),
+                        }],
                         span: Span::unknown(),
                     }),
                     span: Span::unknown(),
                 },
-                ASTNode::Break {
+                // The if-else pattern
+                ASTNode::If {
+                    condition: Box::new(ASTNode::Variable {
+                        name: "is_ws".to_string(),
+                        span: Span::unknown(),
+                    }),
+                    then_body: vec![ASTNode::Assignment {
+                        target: Box::new(ASTNode::Variable {
+                            name: "p".to_string(),
+                            span: Span::unknown(),
+                        }),
+                        value: Box::new(ASTNode::BinaryOp {
+                            operator: BinaryOperator::Add,
+                            left: Box::new(ASTNode::Variable {
+                                name: "p".to_string(),
+                                span: Span::unknown(),
+                            }),
+                            right: Box::new(ASTNode::Literal {
+                                value: LiteralValue::Integer(1),
+                                span: Span::unknown(),
+                            }),
+                            span: Span::unknown(),
+                        }),
+                        span: Span::unknown(),
+                    }],
+                    else_body: Some(vec![ASTNode::Break {
+                        span: Span::unknown(),
+                    }]),
                     span: Span::unknown(),
                 },
             ],
@@ -602,16 +800,36 @@ mod tests {
         let result = canonicalize_loop_expr(&loop_node);
         assert!(result.is_ok());
 
-        let (_, decision) = result.unwrap();
-        assert!(decision.is_fail_fast());
-        assert!(decision.notes[0].contains("Only single-statement loops supported"));
+        let (skeleton, decision) = result.unwrap();
+
+        // Verify success
+        assert!(decision.is_success());
+        assert_eq!(decision.chosen, Some(LoopPatternKind::Pattern3IfPhi));
+
+        // Verify skeleton has Body step
+        assert_eq!(skeleton.steps.len(), 3); // HeaderCond + Body + Update
+        assert!(matches!(
+            skeleton.steps[0],
+            SkeletonStep::HeaderCond { .. }
+        ));
+        assert!(matches!(skeleton.steps[1], SkeletonStep::Body { .. }));
+        assert!(matches!(
+            skeleton.steps[2],
+            SkeletonStep::Update { .. }
+        ));
+
+        // Verify body contains 1 statement
+        match &skeleton.steps[1] {
+            SkeletonStep::Body { stmts } => assert_eq!(stmts.len(), 1),
+            _ => panic!("Expected Body step"),
+        }
     }
 
     #[test]
-    fn test_canonicalize_rejects_if_without_else() {
-        use crate::ast::LiteralValue;
+    fn test_skip_whitespace_fails_without_else() {
+        use crate::ast::{BinaryOperator, LiteralValue};
 
-        // Build loop with if (no else)
+        // Build pattern without else branch (should fail)
         let loop_node = ASTNode::Loop {
             condition: Box::new(ASTNode::Literal {
                 value: LiteralValue::Bool(true),
@@ -622,7 +840,25 @@ mod tests {
                     value: LiteralValue::Bool(true),
                     span: Span::unknown(),
                 }),
-                then_body: vec![],
+                then_body: vec![ASTNode::Assignment {
+                    target: Box::new(ASTNode::Variable {
+                        name: "p".to_string(),
+                        span: Span::unknown(),
+                    }),
+                    value: Box::new(ASTNode::BinaryOp {
+                        operator: BinaryOperator::Add,
+                        left: Box::new(ASTNode::Variable {
+                            name: "p".to_string(),
+                            span: Span::unknown(),
+                        }),
+                        right: Box::new(ASTNode::Literal {
+                            value: LiteralValue::Integer(1),
+                            span: Span::unknown(),
+                        }),
+                        span: Span::unknown(),
+                    }),
+                    span: Span::unknown(),
+                }],
                 else_body: None, // No else branch
                 span: Span::unknown(),
             }],
@@ -634,6 +870,58 @@ mod tests {
 
         let (_, decision) = result.unwrap();
         assert!(decision.is_fail_fast());
-        assert!(decision.notes[0].contains("must have else branch"));
+        assert!(decision
+            .notes[0]
+            .contains("does not match skip_whitespace pattern"));
     }
+
+    #[test]
+    fn test_skip_whitespace_fails_with_wrong_delta() {
+        use crate::ast::{BinaryOperator, LiteralValue};
+
+        // Build pattern with wrong update (p = p - 1 instead of p = p + 1)
+        let loop_node = ASTNode::Loop {
+            condition: Box::new(ASTNode::Literal {
+                value: LiteralValue::Bool(true),
+                span: Span::unknown(),
+            }),
+            body: vec![ASTNode::If {
+                condition: Box::new(ASTNode::Literal {
+                    value: LiteralValue::Bool(true),
+                    span: Span::unknown(),
+                }),
+                then_body: vec![ASTNode::Assignment {
+                    target: Box::new(ASTNode::Variable {
+                        name: "p".to_string(),
+                        span: Span::unknown(),
+                    }),
+                    value: Box::new(ASTNode::BinaryOp {
+                        operator: BinaryOperator::Subtract, // Wrong operator
+                        left: Box::new(ASTNode::Variable {
+                            name: "p".to_string(),
+                            span: Span::unknown(),
+                        }),
+                        right: Box::new(ASTNode::Literal {
+                            value: LiteralValue::Integer(1),
+                            span: Span::unknown(),
+                        }),
+                        span: Span::unknown(),
+                    }),
+                    span: Span::unknown(),
+                }],
+                else_body: Some(vec![ASTNode::Break {
+                    span: Span::unknown(),
+                }]),
+                span: Span::unknown(),
+            }],
+            span: Span::unknown(),
+        };
+
+        let result = canonicalize_loop_expr(&loop_node);
+        assert!(result.is_ok());
+
+        let (_, decision) = result.unwrap();
+        assert!(decision.is_fail_fast());
+    }
+
 }