Backport "Fix superType of SuperType" to LTS (#18942)

Backports #17574 to the LTS branch.

PR submitted by the release tooling.

Author: Kordyjan

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -3037,7 +3037,8 @@ object Types {
   abstract case class SuperType(thistpe: Type, supertpe: Type) extends CachedProxyType with SingletonType {
     override def underlying(using Context): Type = supertpe
     override def superType(using Context): Type =
-      thistpe.baseType(supertpe.typeSymbol)
+      if supertpe.typeSymbol.exists then thistpe.baseType(supertpe.typeSymbol)
+      else super.superType
     def derivedSuperType(thistpe: Type, supertpe: Type)(using Context): Type =
       if ((thistpe eq this.thistpe) && (supertpe eq this.supertpe)) this
       else SuperType(thistpe, supertpe)

tests/run/i17555.scala

@@ -0,0 +1,17 @@
+class Root {
+  override def toString() = "Root"
+}
+trait A extends Root with B {  }
+trait B {
+   override def toString() = "B"
+}
+case class C() extends A {
+  override def toString() = super.toString()
+}
+class D() extends A, Serializable {
+  override def toString() = super.toString()
+}
+
+@main def Test =
+  assert(C().toString == "B")
+  assert(D().toString == "B")