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

The test case is an illegal inheritance from a primitive value class,
which makes this an illegal value class. Previously, the error was
detected by refchecks, but crashes occured earlier (when generating
synthesized methods) or at the same phase block (in extension methods).

The problem is avoided by moving the test to Namer.

Review by @smarter.

Author: odersky

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
+}