# UNDF: UNDF-2026-000000270
diff --git a/src/compiler/scala/tools/nsc/typechecker/Checkable.scala b/src/compiler/scala/tools/nsc/typechecker/Checkable.scala
index abcdef00..cwe407fix 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Checkable.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Checkable.scala
@@ -98,7 +98,11 @@ trait Checkable {
   def propagateKnownTypes(from: Type, to: Symbol): Type = {
     def tparams  = to.typeParams
     val tvars    = tparams map (p => TypeVar(p))
     val tvarType = appliedType(to, tvars)
+    // CWE-407 fix: convert to.baseClasses to a Set before the outer foreach.
+    // Previously: from.baseClasses foreach { bc => if (to.baseClasses.contains(bc))
+    // was O(M×N) where M=|from.baseClasses|, N=|to.baseClasses| — both are List[Symbol].
+    // Symbol equality is reference identity so Set[Symbol] is O(1) contains.
+    val toBaseSet = to.baseClasses.toSet

-    from.baseClasses foreach { bc => if (to.baseClasses.contains(bc)){
+    from.baseClasses foreach { bc => if (toBaseSet.contains(bc)){
       val tps1 = (from baseType bc).typeArgs
       val tps2 = (tvarType baseType bc).typeArgs