From 6ff596d4f8fec289c827a5be2edd6646e6a20bb4 Mon Sep 17 00:00:00 2001
From: ishowta <kinironote@gmail.com>
Date: Wed, 21 Nov 2018 19:56:38 +0900
Subject: [PATCH] Empty check in shallow [backport] (#9676)

---
 lib/system.nim     |  1 +
 tests/seq/tseq.nim | 24 ++++++++++++++++++++++++
 2 files changed, 25 insertions(+)

diff --git a/lib/system.nim b/lib/system.nim
index ace3f5e385..a7b22a2839 100644
--- a/lib/system.nim
+++ b/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
diff --git a/tests/seq/tseq.nim b/tests/seq/tseq.nim
index 6528d518e8..1cb94b308d 100644
--- a/tests/seq/tseq.nim
+++ b/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: