prependMoved failed at compile time

[ringabout]

When diving into #17126, found this issue.

failed at Nim devel

import std/[lists, sequtils]

template main() =
  block:
    var
      l0 = initSinglyLinkedList[int]()
      l1 = [1].toSinglyLinkedList
      l2 = [2, 3].toSinglyLinkedList
      l3 = [4, 5, 6].toSinglyLinkedList
    l3.prependMoved l0
    l2.prependMoved l1

    doAssert l3.toSeq == [4, 5, 6]
    doAssert l2.toSeq == [1, 2, 3]
    l3.prepend l0
    doAssert l3.toSeq == [4, 5, 6]

static: main()
main()

Error: unhandled exception: D:\QQPCmgr\Desktop\Nim\test.nim(16, 14) l3gensym0.toSeq == [4, 5, 6]` [AssertionDefect]

Work at RT

> nim -v
Nim Compiler Version 1.5.1 [Windows: amd64]
Compiled at 2021-02-16