2 files changed, 46 insertions, 0 deletions
diff --git a/chapter16/src/main/java/com/seaofnodes/simple/node/ScopeNode.java b/chapter16/src/main/java/com/seaofnodes/simple/node/ScopeNode.java
index 11e54d82..02baeb8a 100644
--- a/chapter16/src/main/java/com/seaofnodes/simple/node/ScopeNode.java
+++ b/chapter16/src/main/java/com/seaofnodes/simple/node/ScopeNode.java
@@ -268,6 +268,13 @@ public class ScopeNode extends ScopeMinNode {
     // This Scope looks for direct variable uses, or certain simple
     // combinations, and replaces the variable with the upcast variant.
     public Node upcast( Node ctrl, Node pred, boolean invert ) {
+        // 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.
+            upcast(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());
diff --git a/chapter16/src/test/java/com/seaofnodes/simple/Chapter10Test.java b/chapter16/src/test/java/com/seaofnodes/simple/Chapter10Test.java
index 2a33031d..ccc09dcb 100644
--- a/chapter16/src/test/java/com/seaofnodes/simple/Chapter10Test.java
+++ b/chapter16/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(