Defend against AliasingBounds

Author: som-snytt

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

@@ -1166,7 +1166,9 @@ object RefChecks {
       def targetOfHiddenExtension: Symbol =
         val target =
           val target0 = explicitInfo.firstParamTypes.head // required for extension method, the putative receiver
-          target0.dealiasKeepOpaques.typeSymbol.info
+          target0.dealiasKeepOpaques.typeSymbol.info match
+          case tb: AliasingBounds => tb.alias
+          case tp => tp
         val member = target.nonPrivateMember(sym.name)
           .filterWithPredicate: member =>
             member.symbol.isPublic && memberHidesMethod(member)