followup for #16717: minimized example + improved comment (#16721)

lib/system/assign.nim

@@ -122,8 +122,10 @@ proc genericAssignAux(dest, src: pointer, mt: PNimType, shallow: bool) =
       #     var tbObj = TB(p)
       #     tbObj of TC # needs to be false!
       #c_fprintf(stdout, "%s %s\n", pint[].name, mt.name)
-      let srcType = cast[ptr PNimType](src)[] # object is not initialized properly(for example std/times.DateTime)
+      let srcType = cast[ptr PNimType](src)[]
       if srcType != nil:
+        # `!= nil` needed because of cases where object is not initialized properly (see bug #16706)
+        # note that you can have `srcType == nil` yet `src != nil`
         chckObjAsgn(srcType, mt)
       pint[] = mt # cast[ptr PNimType](src)[]
   of tyTuple:

tests/assign/tobject_assign.nim

@@ -1,6 +1,18 @@
-import std/[options, tables, times]
+# bug #16706
 
-type
+block: # reduced example
+  type
+    A = object of RootObj
+      a0: string
+    B = object
+      b0: seq[A]
+  var c = newSeq[A](2)
+  var d = B(b0: c)
+
+when true: # original example
+  import std/[options, tables, times]
+
+  type
     Data* = object
       shifts*: OrderedTable[int64, Shift]
       balance*: float
@@ -16,7 +28,7 @@ type
       qty: Option[float]
       id*: int64
 
-let shift = Shift(
+  let shift = Shift(
     quoted: true,
     date: parse("2000-01-01", "yyyy-MM-dd"),
     description: "abcdef",
@@ -26,12 +38,12 @@ let shift = Shift(
     rate: 462.11,
     qty: some(10.0),
     id: getTime().toUnix()
-)
+  )
 
-var shifts: OrderedTable[int64, Shift]
+  var shifts: OrderedTable[int64, Shift]
-shifts[shift.id] = shift
+  shifts[shift.id] = shift
 
-discard Data(
+  discard Data(
     shifts: shifts,
     balance: 0.00
-)
+  )