fixes #3804

tests/vm/twrong_concat.nim

@@ -0,0 +1,28 @@
+discard """
+  output: '''success'''
+"""
+
+# bug #3804
+
+#import sequtils
+
+type AnObj = ref object
+  field: string
+
+#proc aBug(objs: seq[AnObj]) {.compileTime.} =
+#  discard objs.mapIt(it.field & " bug")
+
+proc sameBug(objs: seq[AnObj]) {.compileTime.} =
+  var strSeq = newSeq[string](objs.len)
+  strSeq[0] = objs[0].field & " bug"
+
+static:
+  var objs: seq[AnObj] = @[]
+  objs.add(AnObj(field: "hello"))
+
+  sameBug(objs)
+  # sameBug(objs)
+  echo objs[0].field
+  assert(objs[0].field == "hello") # fails, because (objs[0].field == "hello bug") - mutated!
+
+echo "success"