datalog: diagonal query (repeated var) regression test (223/223)

lib/datalog/tests/eval.sx

@@ -273,6 +273,16 @@
           (dl-saturated? db))
         false)
 
+      ;; Repeated variable (diagonal): p(X, X) only matches tuples
+      ;; whose two args are equal. The unifier handles this via the
+      ;; subst chain — first occurrence binds X, second occurrence
+      ;; checks against the binding.
+      (dl-et-test-set! "diagonal query"
+        (dl-query
+          (dl-program "p(1, 1). p(2, 3). p(4, 4). p(5, 5).")
+          (list (quote p) (quote X) (quote X)))
+        (list {:X 1} {:X 4} {:X 5}))
+
       ;; A relation can be both EDB-seeded and rule-derived;
       ;; saturate combines facts + derivations.
       (dl-et-test-set! "mixed EDB + IDB same relation"