fixes #15804 (#15820)

* fixes #15804

* fix the existing test

* add the testcase for #15804

Co-authored-by: narimiran <narimiran@disroot.org>
(cherry picked from commit dfd8a83f35)

tests/async/t15804.nim

@@ -0,0 +1,15 @@
+import asyncdispatch
+
+type
+  Foo*[E] = ref object 
+    op: proc(): Future[bool] {.gcsafe.}
+
+proc newFoo*[E](): Foo[E] =
+  result = Foo[E]()
+  result.op = proc(): Future[bool] {.gcsafe,async.} =
+    await sleepAsync(100)
+    result = false
+
+when isMainModule:
+  let f = newFoo[int]()
+  echo waitFor f.op()

tests/async/tasync_noasync.nim

@@ -1,7 +1,7 @@
 discard """
-  errormsg: "undeclared identifier: 'await'"
+  errormsg: "'yield' only allowed in an iterator"
   cmd: "nim c $file"
-  file: "tasync_noasync.nim"
+  file: "asyncmacro.nim"
 """
 import async
 
@@ -12,4 +12,4 @@ await a()
 
 # if we overload a fallback handler to get
 # await only available within {.async.}
-# we would need `{.dirty.}` templates for await
+# we would need `{.dirty.}` templates for await