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.

1 files changed, 10 insertions, 1 deletions

diff --git a/src/dotty/tools/dotc/typer/Namer.scala b/src/dotty/tools/dotc/typer/Namer.scala
index 9da0e2edc..d4aa31498 100644
--- a/src/dotty/tools/dotc/typer/Namer.scala
+++ b/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
+          }
         }
       }