LOGIC-90: <=fd constraint could diverge

The `-relevant?` implementation for `<=fdc` was wrong. If we have two
singleton domains we just check that they are not <= to each
other. However we can also discard the constraint if we have
`interval-<`. Added test case.

src/main/clojure/clojure/core/logic.clj

@@ -3432,10 +3432,11 @@
     IRelevant
     (-relevant? [this s]
       (let-dom s [u du v dv]
-       (cond
-        (not (singleton-dom? du)) true
-        (not (singleton-dom? dv)) true
-        :else (not (<= du dv)))))))
+       (if (and (domain? du) (domain dv))
+         (if (and (singleton-dom? du) (singleton-dom? dv))
+           (not (<= du dv))
+           (not (interval-< du dv)))
+         true)))))
 
 (defn <=fd
   "A finite domain constraint. u must be less than or equal to v.

src/test/clojure/clojure/core/logic/tests.clj

@@ -1356,6 +1356,14 @@
            (firsto r x))
          '([[1 2] 1 2 1]))))
 
+(deftest test-90-<fd-diverge
+  (is (= (run 1 [a b c d]
+           (infd a b c d (interval 0 4))
+           (<fd a b)
+           (<fd c d)
+           (<fd d a))
+         '([2 3 0 1]))))
+
 ;; =============================================================================
 ;; cKanren