close #1550 add testcase (#16640)

tests/iter/t1550.nim

@@ -0,0 +1,20 @@
+type
+  A[T] = iterator(x: T): T {.gcsafe, closure.}
+
+iterator aimp[T](x: T): T {.gcsafe, closure.} =
+  var total = 0
+  while (total < 100):
+    yield total
+    total += x
+
+iterator bimp(y: A[int], z:int): int {.gcsafe, closure.} =
+  for i in y(z):
+    yield i
+
+for x in aimp[int](3):
+  discard x
+
+var y = aimp[int]
+var z = bimp
+for x in z(y, 1):
+  discard x