Empty check in shallow [backport] (#9676)

lib/system.nim

@@ -4017,6 +4017,7 @@ proc shallow*[T](s: var seq[T]) {.noSideEffect, inline.} =
   ## marks a sequence `s` as `shallow`:idx:. Subsequent assignments will not
   ## perform deep copies of `s`. This is only useful for optimization
   ## purposes.
+  if s.len == 0: return
   when not defined(JS) and not defined(nimscript):
     var s = cast[PGenericSeq](s)
     s.reserved = s.reserved or seqShallowFlag

tests/seq/tseq.nim

@@ -170,6 +170,30 @@ block tshallowseq:
   xxx()
 
 
+block tshallowemptyseq:
+  proc test() =
+    var nilSeq: seq[int] = @[]
+    var emptySeq: seq[int] = newSeq[int]()
+    block:
+      var t = @[1,2,3]
+      shallow(nilSeq)
+      t = nilSeq
+      doAssert t == @[]
+    block:
+      var t = @[1,2,3]
+      shallow(emptySeq)
+      t = emptySeq
+      doAssert t == @[]
+    block:
+      var t = @[1,2,3]
+      shallowCopy(t, nilSeq)
+      doAssert t == @[]
+    block:
+      var t = @[1,2,3]
+      shallowCopy(t, emptySeq)
+      doAssert t == @[]
+  test()
+
 
 import strutils
 block ttoseq: