C#: We can't do any reasoning based on unknown types on whether they match in a pattern.

csharp/ql/lib/semmle/code/csharp/controlflow/internal/Completion.qll

@@ -293,6 +293,8 @@ private predicate isMatchingConstant(PatternExpr pe, boolean value) {
   value = true
   or
   exists(Type t, Type strippedType |
+    not t instanceof UnknownType and
+    not strippedType instanceof UnknownType and
     typePatternMustHaveMatchingCompletion(pe, t, strippedType) and
     not typePatternCommonSubType(t, strippedType) and
     value = false