Allow inline to override erased.

Author: odersky

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

@@ -156,8 +156,8 @@ object RefChecks {
    *    1.8.1  M's type is a subtype of O's type, or
    *    1.8.2  M is of type []S, O is of type ()T and S <: T, or
    *    1.8.3  M is of type ()S, O is of type []T and S <: T, or
-   *    1.9.1  If M or O are erased, they must both be erased
-   *    1.9.2  If M or O are extension methods, they must both be extension methods
+   *    1.9.1  If M is erased, O is erased. If O is erased, M is erased or inline.
+   *    1.9.2  If M or O are extension methods, they must both be extension methods.
    *    1.10   If M is an inline or Scala-2 macro method, O cannot be deferred unless
    *           there's also a concrete method that M overrides.
    *    1.11.  If O is a Scala-2 macro, M must be a Scala-2 macro.
@@ -394,7 +394,7 @@ object RefChecks {
         overrideError("must be declared lazy to override a lazy value")
       } else if (member.is(Erased) && !other.is(Erased)) { // (1.9.1)
         overrideError("is erased, cannot override non-erased member")
-      } else if (other.is(Erased) && !member.is(Erased)) { // (1.9.1)
+      } else if (other.is(Erased) && !member.is(Erased | Inline)) { // (1.9.1)
         overrideError("is not erased, cannot override erased member")
       } else if (member.is(Extension) && !other.is(Extension)) { // (1.9.2)
         overrideError("is an extension method, cannot override a normal method")