Several type system fixes

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

@@ -1466,7 +1466,7 @@ object SymDenotations {
       val builder = new BaseDataBuilder
       for (p <- classParents) {
         if (p.typeSymbol.isClass) builder.addAll(p.typeSymbol.asClass.baseClasses)
-        else assert(ctx.mode.is(Mode.Interactive), s"$this has non-class parent: $p")
+        else assert(isRefinementClass || ctx.mode.is(Mode.Interactive), s"$this has non-class parent: $p")
       }
       (classSymbol :: builder.baseClasses, builder.baseClassSet)
     }

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

@@ -989,7 +989,7 @@ object Types {
       case tp: TypeRef =>
         if (tp.symbol.isClass) tp
         else tp.info match {
-          case TypeAlias(tp) => tp.dealias1(keepAnnots): @tailrec
+          case TypeAlias(alias) => alias.dealias1(keepAnnots): @tailrec
           case _ => tp
         }
       case app @ AppliedType(tycon, args) =>
@@ -1967,6 +1967,7 @@ object Types {
     def derivedSelect(prefix: Type)(implicit ctx: Context): Type =
       if (prefix eq this.prefix) this
       else if (prefix.isBottomType) prefix
+      else if (prefix.isInstanceOf[WildcardType]) WildcardType
       else if (isType) {
         val res =
           if (currentSymbol.is(ClassTypeParam)) argForParam(prefix)

compiler/src/dotty/tools/dotc/typer/ProtoTypes.scala

@@ -105,6 +105,7 @@ object ProtoTypes {
     private def hasUnknownMembers(tp: Type)(implicit ctx: Context): Boolean = tp match {
       case tp: TypeVar => !tp.isInstantiated
       case tp: WildcardType => true
+      case NoType => true
       case tp: TypeRef =>
         val sym = tp.symbol
         sym == defn.NothingClass ||

compiler/src/dotty/tools/dotc/typer/RefChecks.scala

@@ -343,7 +343,10 @@ object RefChecks {
         if (autoOverride(member) ||
             other.owner.is(JavaTrait) && ctx.testScala2Mode("`override' modifier required when a Java 8 default method is re-implemented", member.pos))
           member.setFlag(Override)
-        else if (member.owner != clazz && other.owner != clazz && !(other.owner derivesFrom member.owner))
+        else if (member.isType && self.memberInfo(member) =:= self.memberInfo(other))
+          () // OK, don't complain about type aliases which are equal
+        else if (member.owner != clazz && other.owner != clazz &&
+                 !(other.owner derivesFrom member.owner))
           emitOverrideError(
             clazz + " inherits conflicting members:\n  "
               + infoStringWithLocation(other) + "  and\n  " + infoStringWithLocation(member)

tests/neg/typeclass-encoding2.scala

@@ -0,0 +1,26 @@
+// Used to crash with:
+//   assertion failed: class <refinement> has non-class parent
+// Once that was fixed, another crash with:
+//   assertion failed: invalid prefix WildcardType(NoType)
+object runtime1 {
+
+  trait TypeClass1[A] {
+    val common: TypeClassCommon1
+    type This[X] = common.This[X]
+  }
+
+  trait TypeClassCommon1 { self =>
+    type This[X]
+    type Instance[X] <: TypeClass1[X]
+    def inject[A](x: This[A]): Instance[A]// { val common: self.type }
+  }
+
+  trait Extension1[From[_], To[X] <: TypeClass1[X]] extends TypeClassCommon1 {
+    type This[X] = From[X]
+    type Instance[X] = To[X]
+  }
+
+  implicit def inject[A, From[_]](x: From[A])
+      (implicit ev: Extension1[From, _]): ev.Instance[A] { type This[X] = From[X] } =
+    ev.inject(x) // error: found: ev.To[A], required: ev.To[A]{This = From}
+}

tests/pos/this-types.scala

@@ -0,0 +1,16 @@
+trait A {
+  type A_This <: A
+}
+trait B extends A {
+  type A_This = B_This
+  type B_This <: B
+}
+trait C extends A {
+  type A_This = C_This
+  type C_This <: C
+}
+trait D extends B with C {
+  type B_This = D_This
+  type C_This = D_This
+  type D_This <: D
+}