Fix remove on last node of singly-linked list [backport:1.6] (#19353)

changelog.md

@@ -22,6 +22,7 @@
 - `macros.parseExpr` and `macros.parseStmt` now accept an optional
   filename argument for more informative errors.
 - Module `colors` expanded with missing colors from the CSS color standard.
+- Fixed `lists.SinglyLinkedList` being broken after removing the last node ([#19353](https://github.com/nim-lang/Nim/pull/19353)).
 
 ## `std/smtp`
 

lib/pure/collections/lists.nim

@@ -739,6 +739,8 @@ proc remove*[T](L: var SinglyLinkedList[T], n: SinglyLinkedNode[T]): bool {.disc
     if prev.next == nil:
       return false
     prev.next = n.next
+    if L.tail == n:
+      L.tail = prev # update tail if we removed the last node
   true
 
 proc remove*[T](L: var DoublyLinkedList[T], n: DoublyLinkedNode[T]) =

tests/stdlib/tlists.nim

@@ -258,5 +258,18 @@ template main =
     a.add(2)
     doAssert a.toSeq == @[1, 2]
 
+  block RemoveLastNodeFromSinglyLinkedList:
+    var list = initSinglyLinkedList[string]()
+    let n1 = newSinglyLinkedNode("sonic")
+    let n2 = newSinglyLinkedNode("the")
+    let n3 = newSinglyLinkedNode("tiger")
+    let n4 = newSinglyLinkedNode("hedgehog")
+    list.add(n1)
+    list.add(n2)
+    list.add(n3)
+    list.remove(n3)
+    list.add(n4)
+    doAssert list.toSeq == @["sonic", "the", "hedgehog"]
+
 static: main()
 main()