fix #6497 (#16027)

* fix #6497

* add testcase for #16027

compiler/ast.nim

@@ -1441,6 +1441,7 @@ proc copySym*(s: PSym; id: ItemId): PSym =
   result.position = s.position
   result.loc = s.loc
   result.annex = s.annex      # BUGFIX
+  result.constraint = s.constraint
   if result.kind in {skVar, skLet, skField}:
     result.guard = s.guard
     result.bitsize = s.bitsize

tests/ccgbugs/t16027.nim

@@ -0,0 +1,13 @@
+discard """
+  ccodecheck: "__restrict__"
+  action: compile
+  joinable: false
+"""
+
+# see bug #16027
+iterator myitems(s: seq[int]): int =
+  var data {.codegenDecl: "$# __restrict__ $#".} : ptr int = nil
+  yield 1
+
+for i in @[1].myitems:
+  discard