close #2771(add testcase for #2771) (#15932)

(cherry picked from commit 7d51ad96e9)

tests/iter/t2771.nim

@@ -0,0 +1,21 @@
+template t1(i: int): int=
+  i+1
+template t2(i: int): int=
+  i+1
+
+doAssert t1(10).t2() == 12
+
+
+template it1(i: int): iterator(): int =
+  iterator result(): int {.closure, gensym.} =
+    yield i+1
+  result
+
+template it2(iter: iterator(): int): iterator(): int =
+  iterator result(): int {.closure, gensym.} =
+    yield iter()+1
+  result
+
+let x2 = it1(10).it2()
+
+doAssert x2() == 12