[unconstrained] Fix gathering of visited-once varsfix4644

Fixes #4644. This commit fixes an issue where the set `d_unconstrained`
in the unconstrained simplification pass was not computed correctly. The
problem was that visiting the same term multiple times did not remove
the variables appearing in that term from the visited-once set. A simple
example that triggers the issue is the following:

```
(set-logic ALL)
(declare-fun a () Bool)
(declare-fun b () Bool)
(assert (not (= a b)))
(assert (= a (= a b)))
(check-sat)
```

After running `UnconstrainedSimplifier::visitAll()` on both assertions,
we end up with `[b]` as our `d_unconstrained` set. We end up inferring
the substitution `(= a b) --> b` and get `(not b)` and `b`, which is
unsat even though the original problem is sat.

This commit fixes the issue by visiting all the children of a node if we
visit a node for a second time. This makes sure that we remove any
children from the visisted-once set.

1 files changed, 9 insertions, 0 deletions

diff --git a/test/regress/regress0/unconstrained/issue4644.smt2 b/test/regress/regress0/unconstrained/issue4644.smt2
new file mode 100644
index 000000000..aeb2bfc84
--- /dev/null
+++ b/test/regress/regress0/unconstrained/issue4644.smt2
@@ -0,0 +1,9 @@
+; COMMAND-LINE: --unconstrained-simp
+(set-logic ALL)
+(declare-fun a () Bool)
+(declare-fun b () Bool)
+(declare-fun c () Bool)
+(assert (distinct a c))
+(assert (= a b (= b c)))
+(set-info :status sat)
+(check-sat)