From 746889b4ed0f53e433c123ddb2b90dece1298fff Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Thu, 22 Oct 2015 17:05:47 +0200
Subject: Check for cyclic inheritance.

---
 src/dotty/tools/dotc/typer/Namer.scala | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

(limited to 'src/dotty/tools/dotc/typer/Namer.scala')

diff --git a/src/dotty/tools/dotc/typer/Namer.scala b/src/dotty/tools/dotc/typer/Namer.scala
index 3f732f80c..3c491fa7a 100644
--- a/src/dotty/tools/dotc/typer/Namer.scala
+++ b/src/dotty/tools/dotc/typer/Namer.scala
@@ -575,7 +575,14 @@ class Namer { typer: Typer =>
       def checkedParentType(parent: untpd.Tree): Type = {
         val ptype = parentType(parent)(ctx.superCallContext)
         if (cls.isRefinementClass) ptype
-        else checkClassTypeWithStablePrefix(ptype, parent.pos, traitReq = parent ne parents.head)
+        else {
+          val pt = checkClassTypeWithStablePrefix(ptype, parent.pos, traitReq = parent ne parents.head)
+          if (pt.derivesFrom(cls)) {
+            ctx.error(i"cyclic inheritance: $cls extends itself", parent.pos)
+            defn.ObjectClass.typeRef
+          }
+          else pt
+        }
       }
 
       val selfInfo =
-- 
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