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/Chapter10Test.java  | 40 ++++++++++++++++++++++
 1 file changed, 40 insertions(+)

(limited to 'chapter10/src/test/java/com')

diff --git a/chapter10/src/test/java/com/seaofnodes/simple/Chapter10Test.java b/chapter10/src/test/java/com/seaofnodes/simple/Chapter10Test.java
index f2e7ca2c..c39c19b3 100644
--- a/chapter10/src/test/java/com/seaofnodes/simple/Chapter10Test.java
+++ b/chapter10/src/test/java/com/seaofnodes/simple/Chapter10Test.java
@@ -12,6 +12,46 @@ 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 point = new Point;
+        point.x = 42;
+        Point? p = null;
+        if (arg) p = point;
+        """;
+
+    @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; else 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 (!!point) 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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