Fixes #7985

tests/async/t7985.nim

@@ -0,0 +1,19 @@
+discard """
+  file: "t7985.nim"
+  exitcode: 0
+  output: "(value: 1)"
+"""
+import json, asyncdispatch
+
+proc getData(): Future[JsonNode] {.async.} =
+  result = %*{"value": 1}
+
+type
+  MyData = object
+    value: BiggestInt
+
+proc main() {.async.} =
+  let data = to(await(getData()), MyData)
+  echo data
+
+waitFor(main())