Fix validCtor

Author: odersky

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

@@ -5965,8 +5965,15 @@ object Types extends TypeUtils {
           case _ => false
         def validCtor(cls: Symbol): Boolean =
           val ctor = cls.primaryConstructor
-          (!ctor.exists || zeroParams(ctor.info)) // `ContextFunctionN` does not have constructors
-          && (!cls.is(Trait) || validCtor(cls.info.parents.head.classSymbol))
+          if !ctor.exists then true // `ContextFunctionN` does not have constructors
+          else if !zeroParams(ctor.info) then false
+          else if !cls.is(Trait) then true
+          else
+            val firstParentCls = cls.info.parents.head.classSymbol
+            if firstParentCls.isClass && !firstParentCls.is(Trait) then
+              validCtor(firstParentCls) // need to check class constructor as well
+            else
+              true
         val validCtorNEW = validCtor(tp.cls)
 
         if validCtorNEW != validCtorOLD then