Fixed #18838 (#18841) [backport]

compiler/semtypinst.nim

@@ -344,7 +344,11 @@ proc handleGenericInvocation(cl: var TReplTypeVars, t: PType): PType =
       x = lookupTypeVar(cl, x)
       if x != nil:
         if header == t: header = instCopyType(cl, t)
-        header[i] = x
+        header[i] =
+          if x.kind == tyGenericInst:
+            t[i]
+          else:
+            x
         propagateToOwner(header, x)
     else:
       propagateToOwner(header, x)

tests/generics/trecursivegenerics.nim

@@ -1,6 +1,7 @@
 block: # Replicates #18728
   type
     FlipFlop[A, B] = ref object
+      val: A
       next: FlipFlop[B, A]
   
     Trinary[A, B, C] = ref object
@@ -9,4 +10,37 @@ block: # Replicates #18728
   assert typeof(FlipFlop[int, string]().next) is FlipFlop[string, int]
   assert typeof(FlipFlop[string, int]().next) is FlipFlop[int, string]
   assert typeof(Trinary[int, float, string]().next) is Trinary[float, string, int]
-  assert typeof(Trinary[int, float, string]().next.next) is Trinary[string, int, float]
+  assert typeof(Trinary[int, float, string]().next.next) is Trinary[string, int, float]
+  var a = FlipFlop[int, string](val: 100, next: FlipFlop[string, int](val: "Hello"))
+  assert a.val == 100
+  assert a.next.val == "Hello"
+
+block: # 18838
+  type
+    DoublyLinkedNodeObj[T] = object
+      value: T
+
+    DoublyLinkedNode[T] = ref DoublyLinkedNodeObj[T]
+
+    Item[T] = ref object
+      link: DoublyLinkedNode[Item[T]]
+
+    Box = object
+
+  proc newDoublyLinkedNode[T](value: T): DoublyLinkedNode[T] =
+    new(result)
+    result.value = value 
+
+  let link = newDoublyLinkedNode(Item[Box]())
+
+import lists
+block:
+  type
+    Box = object
+    Item[T] = ref object
+      link:DoublyLinkedNode[ Item[T] ]
+
+    ItemSimple = ref object
+      link:DoublyLinkedNode[ ItemSimple ]
+
+  let link = newDoublyLinkedNode( Item[Box]() )