fixes #1691

compiler/ast.nim

@@ -1317,6 +1317,13 @@ proc skipTypes*(t: PType, kinds: TTypeKinds): PType =
   result = t
   while result.kind in kinds: result = lastSon(result)
 
+proc skipTypesOrNil*(t: PType, kinds: TTypeKinds): PType =
+  ## same as skipTypes but handles 'nil'
+  result = t
+  while result != nil and result.kind in kinds:
+    if result.len == 0: return nil
+    result = lastSon(result)
+
 proc isGCedMem*(t: PType): bool {.inline.} =
   result = t.kind in {tyString, tyRef, tySequence} or
            t.kind == tyProc and t.callConv == ccClosure

compiler/semtypes.nim

@@ -646,7 +646,10 @@ proc semObjectNode(c: PContext, n: PNode, prev: PType): PType =
   # n.sons[0] contains the pragmas (if any). We process these later...
   checkSonsLen(n, 3)
   if n.sons[1].kind != nkEmpty:
-    base = skipTypes(semTypeNode(c, n.sons[1].sons[0], nil), skipPtrs)
+    base = skipTypesOrNil(semTypeNode(c, n.sons[1].sons[0], nil), skipPtrs)
+    if base.isNil:
+      localError(n.info, errIllegalRecursionInTypeX, "object")
+    else:
       var concreteBase = skipGenericInvocation(base).skipTypes(skipPtrs)
       if concreteBase.kind == tyObject and tfFinal notin concreteBase.flags:
         addInheritedFields(c, check, pos, concreteBase)

tests/objects/tillegal_recursion.nim

@@ -0,0 +1,7 @@
+discard """
+  errormsg: "illegal recursion in type 'object'"
+  line: 7
+"""
+# bug #1691
+type
+  Foo = ref object of Foo