From bf3e5632ddf21c109f7ef0f61bcc62e2ed0f9d49 Mon Sep 17 00:00:00 2001
From: Cliff Click <cliffc@acm.org>
Date: Sat, 19 Sep 2026 10:30:13 -0700
Subject: Handle double-negation preserving not-null knowledge

---
 .../java/com/seaofnodes/simple/node/ScopeNode.java | 17 ++++++++--
 .../java/com/seaofnodes/simple/Chapter10Test.java  | 39 ++++++++++++++++++++++
 2 files changed, 54 insertions(+), 2 deletions(-)

(limited to 'chapter17/src')

diff --git a/chapter17/src/main/java/com/seaofnodes/simple/node/ScopeNode.java b/chapter17/src/main/java/com/seaofnodes/simple/node/ScopeNode.java
index 86aa7665..e71c669f 100644
--- a/chapter17/src/main/java/com/seaofnodes/simple/node/ScopeNode.java
+++ b/chapter17/src/main/java/com/seaofnodes/simple/node/ScopeNode.java
@@ -287,10 +287,23 @@ public class ScopeNode extends ScopeMinNode {
     // E.g. "if( ptr ) ptr.field;" is legal because ptr is known not-null.
     public void addGuards( Node ctrl, Node pred, boolean invert ) {
         assert ctrl instanceof CFGNode;
-        _guards.add(ctrl);      // Marker to distinguish 0,1,2 guards
+        _guards.add(ctrl);      // Marker between guard sets
         // add pred & its cast to the normal input list, with special Vars
         if( ctrl._type == Type.XCONTROL || pred==null || pred.isDead() )
             return;           // Dead, do not add any guards
+        _addGuards(ctrl,pred,invert);
+    }
+
+    private void _addGuards( Node ctrl, Node pred, boolean invert ) {
+        if( pred==null || pred.isDead() )
+            return;
+        // A single negation is handled below. For !!p (including p != null),
+        // also refine the underlying p without changing the Boolean result.
+        if( pred instanceof NotNode not && not.in(1) instanceof NotNode ) {
+            pred.keep();        // Recursive peepholes may rediscover this Not.
+            _addGuards(ctrl,not.in(1),!invert);
+            pred.unkeep();
+        }
         // Invert the If conditional
         if( invert )
             pred = pred instanceof NotNode not ? not.in(1) : IterPeeps.add(new NotNode(pred).peephole());
@@ -322,7 +335,7 @@ public class ScopeNode extends ScopeMinNode {
     // Remove matching pred/cast pairs from this guarded region.
     public void removeGuards( Node ctrl ) {
         assert ctrl instanceof CFGNode;
-        // 0,1 or 2 guards
+        // Pop the guards up to this region's marker.
         while( true ) {
             Node g = _guards.pop();
             if( g == ctrl ) break;
diff --git a/chapter17/src/test/java/com/seaofnodes/simple/Chapter10Test.java b/chapter17/src/test/java/com/seaofnodes/simple/Chapter10Test.java
index d41c83d8..d6f6bcd3 100644
--- a/chapter17/src/test/java/com/seaofnodes/simple/Chapter10Test.java
+++ b/chapter17/src/test/java/com/seaofnodes/simple/Chapter10Test.java
@@ -8,6 +8,45 @@ import static org.junit.Assert.fail;
 
 public class Chapter10Test {
 
+    // Issue #246: null-check guards start in Chapter 10; arrays arrive in Chapter 15.
+    private static final String NULLABLE_POINT_SOURCE = """
+        struct Point { int x; };
+        Point?[] !points = new Point?[2];
+        points[arg] = new Point { x = 42; };
+        Point? p = points[1];
+        """;
+
+    @Test
+    public void testNullGuards() {
+        for( String body : new String[] {
+            "if (p != null) return p.x; return -1;",
+            "if (null != p) return p.x; return -1;",
+            "if (!!!!p) return p.x; return -1;",
+            "if (!!!p) return -1; return p.x;",
+            "int b = !!p; if (b) return p.x + b - 1; return -1;"
+        } ) {
+            StopNode stop = new Parser(NULLABLE_POINT_SOURCE+body).parse().iterate();
+            assertEquals(body,-1L,com.seaofnodes.simple.evaluator.Evaluator.evaluate(stop,0));
+            assertEquals(body,42L,com.seaofnodes.simple.evaluator.Evaluator.evaluate(stop,1));
+        }
+    }
+
+    @Test
+    public void testNullGuardErrors() {
+        for( String body : new String[] {
+            "return p.x;",
+            "if (!!points) return p.x; return -1;",
+            "if (!!p) { int x = p.x; } return p.x;"
+        } ) {
+            try {
+                new Parser(NULLABLE_POINT_SOURCE+body).parse().iterate();
+                fail(body);
+            } catch( RuntimeException e ) {
+                assertEquals(body,"Might be null accessing 'x'",e.getMessage());
+            }
+        }
+    }
+
     @Test
     public void testFuzzer() {
         Parser parser = new Parser(
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