add testcase for #9091 (#15791)

* add testcase for #9091

* more tests

tests/stdlib/t9091.nim

@@ -0,0 +1,36 @@
+discard """
+  targets:  "c"
+  output:   "test AObj"
+  action:   "compile"
+  exitcode: 0
+  timeout:  60.0
+"""
+import streams
+
+block:
+  type Mine = ref object
+    a: int
+
+  proc write(io: Stream, t: Mine) =
+    io.write("sure")
+
+  let str = newStringStream()
+  let mi = new Mine
+
+  str.write(mi)
+
+block:
+  type
+    AObj = object
+      x: int
+
+  proc foo(a: int): string = ""
+
+  proc test(args: varargs[string, foo]) =
+    echo "varargs"
+
+  proc test(a: AObj) =
+    echo "test AObj"
+
+  let x = AObj()
+  test(x)