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

lib/datalog/scoreboard.json

@@ -1,13 +1,13 @@
 {
   "lang": "datalog",
-  "total_passed": 222,
+  "total_passed": 223,
   "total_failed": 0,
-  "total": 222,
+  "total": 223,
   "suites": [
     {"name":"tokenize","passed":26,"failed":0,"total":26},
     {"name":"parse","passed":18,"failed":0,"total":18},
     {"name":"unify","passed":28,"failed":0,"total":28},
-    {"name":"eval","passed":29,"failed":0,"total":29},
+    {"name":"eval","passed":30,"failed":0,"total":30},
     {"name":"builtins","passed":19,"failed":0,"total":19},
     {"name":"semi_naive","passed":8,"failed":0,"total":8},
     {"name":"negation","passed":10,"failed":0,"total":10},
@@ -16,5 +16,5 @@
     {"name":"magic","passed":24,"failed":0,"total":24},
     {"name":"demo","passed":21,"failed":0,"total":21}
   ],
-  "generated": "2026-05-08T10:31:43+00:00"
+  "generated": "2026-05-08T10:33:34+00:00"
 }

lib/datalog/scoreboard.md

@@ -1,13 +1,13 @@
 # datalog scoreboard
 
-**222 / 222 passing** (0 failure(s)).
+**223 / 223 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
 | tokenize | 26 | 26 | ok |
 | parse | 18 | 18 | ok |
 | unify | 28 | 28 | ok |
-| eval | 29 | 29 | ok |
+| eval | 30 | 30 | ok |
 | builtins | 19 | 19 | ok |
 | semi_naive | 8 | 8 | ok |
 | negation | 10 | 10 | ok |

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"