add deref that wraps original `system.[]

lib/std/wrapnils.nim

@@ -25,17 +25,18 @@ proc wrapnil*[T](a: T): Wrapnil[T] =
 template `.`*(a: Wrapnil, b): untyped =
   let a1 = a # to avoid double evaluations
   let a2 = a1.valueImpl
+  type T = Wrapnil[type(a2.b)]
   if a1.validImpl:
     when type(a2) is ref|ptr:
       if a2 == nil:
-        default(Wrapnil[type(a2.b)])
+        default(T)
       else:
         wrapnil(a2.b)
     else:
       wrapnil(a2.b)
   else:
     # nil is "sticky"; this is needed, see tests
-    default(Wrapnil[type(a2.b)])
+    default(T)
 
 {.pop.}
 
@@ -51,3 +52,16 @@ template `[]`*[I](a: Wrapnil, i: I): untyped =
     wrapnil(a1.valueImpl[i])
   else:
     default(Wrapnil[type(a1.valueImpl[i])])
+
+template deref*(a: Wrapnil): untyped =
+  ## Since `[]` is hijacked, we can use `deref` that wraps original `system.[]`
+  let a1 = a # to avoid double evaluations
+  let a2 = a1.valueImpl
+  type T = Wrapnil[type(a2[])]
+  if a1.validImpl:
+    if a2 == nil:
+      default(T)
+    else:
+      wrapnil(a2[])
+  else:
+    default(T)

tests/stdlib/twrapnils.nim

@@ -10,6 +10,8 @@ var witness = 0
 proc main() =
   type Bar = object
     b1: int
+    b2: ptr string
+
   type Foo = ref object
     x1: float
     x2: Foo
@@ -19,6 +21,9 @@ proc main() =
     x6: ptr Bar
     x7: array[2, string]
     x8: seq[int]
+    x9: ref Bar
+
+  proc fun(a: Bar): auto = a.b2
 
   var a: Foo
   var x6 = create(Bar)
@@ -57,4 +62,8 @@ proc main() =
   doAssert initFoo(1.3).wrapnil.x1[] == 1.3
   doAssert witness == 1
 
+  # since `[]` is hijacked, we can use `deref` that wraps original `system.[]`
+  # here, it's used twice, to deref `ref Bar` and then `ptr string`
+  doAssert a.wrapnil.x9.deref.fun.deref[] == ""
+
 main()