datalog: group-by-via-aggregate-in-rule test (216/216)

lib/datalog/scoreboard.json

@@ -1,8 +1,8 @@
 {
   "lang": "datalog",
-  "total_passed": 215,
+  "total_passed": 216,
   "total_failed": 0,
-  "total": 215,
+  "total": 216,
   "suites": [
     {"name":"tokenize","passed":26,"failed":0,"total":26},
     {"name":"parse","passed":18,"failed":0,"total":18},
@@ -11,10 +11,10 @@
     {"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},
-    {"name":"aggregates","passed":18,"failed":0,"total":18},
+    {"name":"aggregates","passed":19,"failed":0,"total":19},
     {"name":"api","passed":20,"failed":0,"total":20},
     {"name":"magic","passed":22,"failed":0,"total":22},
     {"name":"demo","passed":18,"failed":0,"total":18}
   ],
-  "generated": "2026-05-08T10:19:48+00:00"
+  "generated": "2026-05-08T10:21:54+00:00"
 }

lib/datalog/scoreboard.md

@@ -1,6 +1,6 @@
 # datalog scoreboard
 
-**215 / 215 passing** (0 failure(s)).
+**216 / 216 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
@@ -11,7 +11,7 @@
 | builtins | 19 | 19 | ok |
 | semi_naive | 8 | 8 | ok |
 | negation | 10 | 10 | ok |
-| aggregates | 18 | 18 | ok |
+| aggregates | 19 | 19 | ok |
 | api | 20 | 20 | ok |
 | magic | 22 | 22 | ok |
 | demo | 18 | 18 | ok |

lib/datalog/tests/aggregates.sx

@@ -227,6 +227,22 @@
         (list {:L (list)}))
 
       ;; Aggregate vs single distinct.
+      ;; Group-by via aggregate-in-rule-body. Per-user friend count
+      ;; over a friends relation. The U var is bound by the prior
+      ;; positive lit u(U) so the aggregate counts only U-rooted
+      ;; friends per group.
+      (dl-at-test-set! "group-by per-user friend count"
+        (dl-query
+          (dl-program
+            "u(alice). u(bob). u(carol).
+             f(alice, x). f(alice, y). f(bob, x).
+             counts(U, N) :- u(U), count(N, X, f(U, X)).")
+          (list (quote counts) (quote U) (quote N)))
+        (list
+          {:U (quote alice) :N 2}
+          {:U (quote bob) :N 1}
+          {:U (quote carol) :N 0}))
+
       ;; Stratification: recursion through aggregation is rejected.
       (dl-at-test! "agg recursion rejected"
         (dl-at-throws?