Always introduce fresh variable for unconstrained APPLY_UF (#4472)

Fixes an unsoundness in unconstrained simplification, fixes #4469.

1 files changed, 7 insertions, 4 deletions

diff --git a/src/preprocessing/passes/unconstrained_simplifier.cpp b/src/preprocessing/passes/unconstrained_simplifier.cpp
index 8a2c58b99..7cf6a79bd 100644
--- a/src/preprocessing/passes/unconstrained_simplifier.cpp
+++ b/src/preprocessing/passes/unconstrained_simplifier.cpp
@@ -577,10 +577,9 @@ void UnconstrainedSimplifier::processUnconstrained()
             {
               currentSub = current;
             }
-            if (parent.getType() != current.getType())
-            {
-              currentSub = newUnconstrainedVar(parent.getType(), currentSub);
-            }
+            // always introduce a new variable; it is unsound to try to reuse
+            // currentSub as the variable, see issue #4469.
+            currentSub = newUnconstrainedVar(parent.getType(), currentSub);
             current = parent;
           }
           else
@@ -779,6 +778,10 @@ void UnconstrainedSimplifier::processUnconstrained()
     }
     if (!currentSub.isNull())
     {
+      Trace("unc-simp")
+          << "UnconstrainedSimplifier::processUnconstrained: introduce "
+          << currentSub << " for " << current << ", parent " << parent
+          << std::endl;
       Assert(currentSub.isVar());
       d_substitutions.addSubstitution(current, currentSub, false);
     }