From afa4bc34b41307c6c63e8c7c973afe295e9fedd8 Mon Sep 17 00:00:00 2001
From: rockcavera <rockcavera@gmail.com>
Date: Thu, 30 Dec 2021 17:52:48 -0300
Subject: [PATCH] Fix #19297 - fixing broken list after adding empty list
 (#19299)

* Update lists.nim

* Update tlists.nim

* removed check `if b.tail != nil`

The tail of the list being null it is still possible to retrieve its end by going through all nodes from the head. So checking for null from `b.tail` is unnecessary. However, setting `a.tail = b.tail` only if `a.head != nil`, so you don't break a good list with an already broken one.

(cherry picked from commit dc5c88ca79e871aefa76c7fefae4282c639a3283)
---
 lib/pure/collections/lists.nim | 11 ++++++-----
 tests/stdlib/tlists.nim        | 12 ++++++++++++
 2 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/lib/pure/collections/lists.nim b/lib/pure/collections/lists.nim
index af53bb68b4..25ba1be028 100644
--- a/lib/pure/collections/lists.nim
+++ b/lib/pure/collections/lists.nim
@@ -531,11 +531,12 @@ proc addMoved*[T](a, b: var SinglyLinkedList[T]) {.since: (1, 5, 1).} =
         ci
     assert s == [0, 1, 0, 1, 0, 1]
 
-  if a.tail != nil:
-    a.tail.next = b.head
-  a.tail = b.tail
-  if a.head == nil:
-    a.head = b.head
+  if b.head != nil:
+    if a.head == nil:
+      a.head = b.head
+    else:
+      a.tail.next = b.head
+    a.tail = b.tail
   if a.addr != b.addr:
     b.head = nil
     b.tail = nil
diff --git a/tests/stdlib/tlists.nim b/tests/stdlib/tlists.nim
index 54fa8b24fd..2f6d5ee6a5 100644
--- a/tests/stdlib/tlists.nim
+++ b/tests/stdlib/tlists.nim
@@ -233,6 +233,18 @@ template main =
     doAssert l.toSeq == [1]
     doAssert l.remove(l.head) == true
     doAssert l.toSeq == []
+  
+  block issue19297: # add (appends a shallow copy)
+    var a: SinglyLinkedList[int]
+    var b: SinglyLinkedList[int]
+
+    doAssert a.toSeq == @[]
+    a.add(1)
+    doAssert a.toSeq == @[1]
+    a.add(b)
+    doAssert a.toSeq == @[1]
+    a.add(2)
+    doAssert a.toSeq == @[1, 2]
 
 static: main()
 main()