fix(template-engine): collision detection sees all slots of a repeatedly-composed template

Claude-Session: https://claude.ai/code/session_01MtdgwpEeCUn6cUA5f1LMPj

src/ZB.MOM.WW.ScadaBridge.TemplateEngine/CollisionDetector.cs

@@ -123,24 +123,37 @@ public static class CollisionDetector
         List<ResolvedMember> members,
         HashSet<int> visited)
     {
+        // `visited` is the current RECURSION PATH, not a global visited-set: an id is
+        // added on entry and REMOVED on exit (finally), so a template legitimately
+        // composed into more than one slot is re-descended for each slot. The early
+        // return fires only on a genuine cycle — an id already on the ACTIVE path.
+        // (A global visited-set would blind the detector to every collision reachable
+        // only through the second-and-later slot of a repeatedly-composed template.)
         if (!visited.Add(template.Id))
             return;
 
-        // Add direct members of this composed template with the prefix. Inherited
-        // placeholder rows are KEPT here: the composed-module walk does not climb the
-        // composed template's parent chain, so the placeholders are the only
-        // representation of a derived module's inherited members.
-        CollectDirectMembers(template, prefix, $"module '{prefix}'", members, skipInherited: false);
-
-        // Recurse into nested compositions
-        foreach (var composition in template.Compositions)
+        try
         {
-            if (lookup.TryGetValue(composition.ComposedTemplateId, out var nested))
+            // Add direct members of this composed template with the prefix. Inherited
+            // placeholder rows are KEPT here: the composed-module walk does not climb the
+            // composed template's parent chain, so the placeholders are the only
+            // representation of a derived module's inherited members.
+            CollectDirectMembers(template, prefix, $"module '{prefix}'", members, skipInherited: false);
+
+            // Recurse into nested compositions
+            foreach (var composition in template.Compositions)
             {
-                var nestedPrefix = $"{prefix}.{composition.InstanceName}";
-                CollectComposedMembers(nested, nestedPrefix, lookup, members, visited);
+                if (lookup.TryGetValue(composition.ComposedTemplateId, out var nested))
+                {
+                    var nestedPrefix = $"{prefix}.{composition.InstanceName}";
+                    CollectComposedMembers(nested, nestedPrefix, lookup, members, visited);
+                }
             }
         }
+        finally
+        {
+            visited.Remove(template.Id);
+        }
     }
 
     private static void CollectInheritedMembers(

tests/ZB.MOM.WW.ScadaBridge.TemplateEngine.Tests/CollisionDetectorTests.cs

@@ -188,4 +188,64 @@ public class CollisionDetectorTests
         Assert.NotEmpty(collisions);
         Assert.Contains(collisions, c => c.Contains("dup.Shared"));
     }
+
+    // ========================================================================
+    // Task 10: repeated composition blindness. A template composed into TWO
+    // different slots was added to the global visited-set on the first slot, so
+    // the SECOND slot's subtree was never collected — the detector went blind to
+    // any collision reachable only through the second slot. The visited-set must
+    // be a recursion PATH set (removed on the way back out), not a global set, so
+    // siblings/other slots can re-descend the same template.
+    // ========================================================================
+
+    [Fact]
+    public void DetectCollisions_TemplateComposedTwiceWithinSubtree_SeesCollisionUnderSecondSlot()
+    {
+        // Z carries the leaf member "Val".
+        var z = new Template("Z") { Id = 4 };
+        z.Attributes.Add(new TemplateAttribute("Val") { Id = 40, TemplateId = 4, DataType = DataType.Float });
+
+        // Y composes Z as "z".
+        var y = new Template("Y") { Id = 3 };
+        y.Compositions.Add(new TemplateComposition("z") { Id = 1, TemplateId = 3, ComposedTemplateId = 4 });
+
+        // W composes Y into TWO slots (y1, y2). Because both slots are visited within
+        // the SAME CollectComposedMembers walk, they share one visited set — the
+        // second slot's subtree was previously never collected.
+        var w = new Template("W") { Id = 2 };
+        w.Compositions.Add(new TemplateComposition("y1") { Id = 2, TemplateId = 2, ComposedTemplateId = 3 });
+        w.Compositions.Add(new TemplateComposition("y2") { Id = 3, TemplateId = 2, ComposedTemplateId = 3 });
+
+        // X composes W as "w" AND declares a direct attribute whose path collides with
+        // a member reachable only via the SECOND slot: "w.y2.z.Val".
+        var x = new Template("X") { Id = 1 };
+        x.Attributes.Add(new TemplateAttribute("w.y2.z.Val") { Id = 10, TemplateId = 1, DataType = DataType.Float });
+        x.Compositions.Add(new TemplateComposition("w") { Id = 4, TemplateId = 1, ComposedTemplateId = 2 });
+
+        var all = new List<Template> { x, w, y, z };
+        var collisions = CollisionDetector.DetectCollisions(x, all);
+
+        // "w.y2.z.Val" exists both as X's direct attribute and via the w→y2→z→Val subtree.
+        Assert.NotEmpty(collisions);
+        Assert.Contains(collisions, c => c.Contains("w.y2.z.Val"));
+    }
+
+    [Fact]
+    public void DetectCollisions_CyclicComposition_DoesNotLoopForever()
+    {
+        // Genuine cycle guard: A composes B, B composes A. The recursion-path set
+        // must still short-circuit a template already on the ACTIVE path so the
+        // walk terminates instead of recursing infinitely.
+        var a = new Template("A") { Id = 1 };
+        a.Compositions.Add(new TemplateComposition("b") { Id = 1, TemplateId = 1, ComposedTemplateId = 2 });
+
+        var b = new Template("B") { Id = 2 };
+        b.Compositions.Add(new TemplateComposition("a") { Id = 2, TemplateId = 2, ComposedTemplateId = 1 });
+
+        var all = new List<Template> { a, b };
+
+        // Must return (terminate) rather than stack-overflow / hang.
+        var collisions = CollisionDetector.DetectCollisions(a, all);
+        Assert.Empty(collisions);
+    }
 }