fixes bug reported in PR #5637

compiler/ast.nim

@@ -1387,6 +1387,14 @@ proc skipTypes*(t: PType, kinds: TTypeKinds): PType =
   result = t
   while result.kind in kinds: result = lastSon(result)
 
+proc skipTypes*(t: PType, kinds: TTypeKinds; maxIters: int): PType =
+  result = t
+  var i = maxIters
+  while result.kind in kinds:
+    result = lastSon(result)
+    dec i
+    if i == 0: return nil
+
 proc skipTypesOrNil*(t: PType, kinds: TTypeKinds): PType =
   ## same as skipTypes but handles 'nil'
   result = t

compiler/semtypes.nim

@@ -1167,7 +1167,10 @@ proc semGeneric(c: PContext, n: PNode, s: PSym, prev: PType): PType =
 
   # special check for generic object with
   # generic/partial specialized parent
-  let tx = result.skipTypes(abstractPtrs)
+  let tx = result.skipTypes(abstractPtrs, 50)
+  if tx.isNil:
+    localError(n.info, "invalid recursion in type '$1'" % typeToString(result[0]))
+    return errorType(c)
   if tx != result and tx.kind == tyObject and tx.sons[0] != nil:
     semObjectTypeForInheritedGenericInst(c, n, tx)
 

tests/types/tillegaltyperecursion2.nim

@@ -0,0 +1,8 @@
+discard """
+  errormsg: "invalid recursion in type 'Executor'"
+  line: 8
+"""
+# bug reported by PR #5637
+type
+  Executor[N] = Executor[N]
+var e: Executor[int]