Disallow abstract methods in extensions

Author: odersky

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -3484,6 +3484,8 @@ object Parsers {
         else if tparams.nonEmpty && stat.tparams.nonEmpty then
           syntaxError(i"extension method cannot have type parameters since some were already given previously",
             stat.tparams.head.span)
+        else if stat.rhs.isEmpty then
+          syntaxError(i"extension method cannot be abstract", stat.span)
       case stat =>
         syntaxError(i"extension clause can only define methods", stat.span)
     }

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

@@ -170,7 +170,7 @@ object Nullables:
       case info :: infos1 =>
         if info.asserted.contains(ref) then true
         else if info.retracted.contains(ref) then false
-        else impliesNotNull(infos1)(ref)
+        else infos1.impliesNotNull(ref)
       case _ =>
         false