fixes #14983 (#15320)

* fixes #14983

* allow bootstrapping with 0.20

* added a test case for the new system.add with a sink parameter

* make npeg green again

lib/system.nim

@@ -1215,6 +1215,32 @@ when defined(nimscript) or not defined(nimSeqsV2):
     ## Generic code becomes much easier to write if the Nim naming scheme is
     ## respected.
 
+when defined(gcDestructors):
+  proc add*[T](x: var seq[T], y: sink openArray[T]) {.noSideEffect.} =
+    ## Generic proc for adding a container `y` to a container `x`.
+    ##
+    ## For containers that have an order, `add` means *append*. New generic
+    ## containers should also call their adding proc `add` for consistency.
+    ## Generic code becomes much easier to write if the Nim naming scheme is
+    ## respected.
+    ##
+    ## See also:
+    ## * `& proc <#&,seq[T][T],seq[T][T]>`_
+    ##
+    ## .. code-block:: Nim
+    ##   var s: seq[string] = @["test2","test2"]
+    ##   s.add("test") # s <- @[test2, test2, test]
+    {.noSideEffect.}:
+      let xl = x.len
+      setLen(x, xl + y.len)
+      for i in 0..high(y):
+        when nimvm:
+          # workaround the fact that the VM does not yet
+          # handle sink parameters properly:
+          x[xl+i] = y[i]
+        else:
+          x[xl+i] = move y[i]
+else:
   proc add*[T](x: var seq[T], y: openArray[T]) {.noSideEffect.} =
     ## Generic proc for adding a container `y` to a container `x`.
     ##
@@ -1234,6 +1260,7 @@ proc add*[T](x: var seq[T], y: openArray[T]) {.noSideEffect.} =
       setLen(x, xl + y.len)
       for i in 0..high(y): x[xl+i] = y[i]
 
+
 when defined(nimSeqsV2):
   template movingCopy(a, b) =
     a = move(b)

tests/destructor/tarc.nim

@@ -8,6 +8,8 @@ Hello
 2
 0
 List
+@["4", "5", "6", "", "", "a", ""]
+@["", "", "a", ""]
 '''
   cmd: '''nim c --gc:arc $file'''
 """
@@ -169,3 +171,14 @@ var a = Tag(kind: List)
 var b = a
 echo a.kind
 var c = a
+
+proc testAdd(i: int; yyy: openArray[string]) =
+  var x: seq[string]
+  x.add [$i, $(i+1), $(i+2)]
+  x.add yyy
+  echo x
+
+var y = newSeq[string](4)
+y[2] = "a"
+testAdd(4, y)
+echo y