Fix #1653: Check "no inherit from final" earlier.

src/dotty/tools/dotc/typer/Namer.scala

@@ -666,6 +666,8 @@ class Namer { typer: Typer =>
        * (1) It must be a class type with a stable prefix (@see checkClassTypeWithStablePrefix)
        * (2) If may not derive from itself
        * (3) Overriding type parameters must be correctly forwarded. (@see checkTypeParamOverride)
+       * (4) The class is not final
+       * (5) If the class is sealed, it is defined in the same compilation unit as the current class
        */
       def checkedParentType(parent: untpd.Tree, paramAccessors: List[Symbol]): Type = {
         val ptype = parentType(parent)(ctx.superCallContext)
@@ -684,7 +686,14 @@ class Namer { typer: Typer =>
           }
           else if (!paramAccessors.forall(checkTypeParamOverride(pt, _)))
             defn.ObjectType
-          else pt
+          else {
+            val pclazz = pt.typeSymbol
+            if (pclazz.is(Final))
+              ctx.error(em"cannot extend final $pclazz", cls.pos)
+            if (pclazz.is(Sealed) && pclazz.associatedFile != cls.associatedFile)
+              ctx.error(em"cannot extend sealed $pclazz in different compilation unit", cls.pos)
+            pt
+          }
         }
       }
 

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

@@ -72,8 +72,7 @@ object RefChecks {
     }
   }
 
-  /** Check that final and sealed restrictions on class parents
-   *  and that self type of this class conforms to self types of parents.
+  /** Check that self type of this class conforms to self types of parents.
    *  and required classes.
    */
   private def checkParents(cls: Symbol)(implicit ctx: Context): Unit = cls.info match {
@@ -83,14 +82,8 @@ object RefChecks {
         if (otherSelf.exists && !(cinfo.selfType <:< otherSelf))
           ctx.error(ex"$category: self type ${cinfo.selfType} of $cls does not conform to self type $otherSelf of $relation ${other.classSymbol}", cls.pos)
       }
-      for (parent <- cinfo.classParents) {
-        val pclazz = parent.classSymbol
-        if (pclazz.is(Final))
-          ctx.error(em"cannot extend final $pclazz", cls.pos)
-        if (pclazz.is(Sealed) && pclazz.associatedFile != cls.associatedFile)
-          ctx.error(em"cannot extend sealed $pclazz in different compilation unit", cls.pos)
+      for (parent <- cinfo.classParents)
         checkSelfConforms(parent, "illegal inheritance", "parent")
-      }
       for (reqd <- cinfo.givenSelfType.classSymbols)
         checkSelfConforms(reqd.typeRef, "missing requirement", "required")
     case _ =>

tests/neg/i1653.scala

@@ -0,0 +1,3 @@
+trait Foo {
+  def foo() = new Unit with Foo  // error
+}

tests/neg/i705-inner-value-class.scala

@@ -15,8 +15,6 @@ object Test {
     class C(val a: Int) extends AnyVal // error: value class may not be a local class
     new C(1)
   }
-  class B1(val b: Int) extends B(b) // error: cannot extend final class B
-//  class D extends B( { class E(val a: Int) extends AnyVal; new E(1) } )
 }
 
 

tests/neg/i705-inner-value-class2.scala

@@ -0,0 +1,9 @@
+class Foo {
+  class B(val a: Int) extends AnyVal
+}
+
+object Test {
+  class B1(val b: Int) extends B(b) // error: cannot extend final class B
+}
+
+