add regression test

tests/unsat/reg_dt.smt2

@@ -0,0 +1,34 @@
+
+; expect: unsat
+
+(set-logic QF_UFDT)
+(declare-datatypes
+  ((foo 0))
+  (((A) (B) (C))))
+
+(declare-datatypes
+  ((tup 0))
+  (((mk_tup (x foo) (y foo)))))
+
+(declare-datatypes
+  ((bar 0))
+  (((mk_bar (b1 tup) (b2 Bool)))))
+
+(declare-fun x0 () bar)
+(declare-fun f (bar) bar)
+
+(assert
+ (= (f x0)
+    (mk_bar (b1 x0) (not (b2 x0)))))
+
+(assert
+ (let ((x1 (f x0)))
+ (= (f x1)
+    (mk_bar (b1 x1) (not (b2 x1))))))
+
+; the goal: f(f(x0))=x0
+(assert (not (= x0 (f (f x0)))))
+
+(check-sat)
+
+