fixes #7637; assignments are not allowed to slice object; minor breaking change

tests/objects/tobjconstr.nim

@@ -9,8 +9,8 @@ discard """
 (k: kindA, a: (x: "abc", z: [1, 8, 3]), method: ())
 (k: kindA, a: (x: "abc", z: [1, 9, 3]), method: ())
 (k: kindA, a: (x: "abc", z: [1, 10, 3]), method: ())
-(x: 123)
-(x: 123)
+(y: 0, x: 123)
+(y: 678, x: 123)
 (z: 89, y: 0, x: 128)
 (y: 678, x: 123)
 (y: 678, x: 123)
@@ -33,7 +33,6 @@ type
       `method`: TEmpty # bug #1791
 
 proc `$`[T](s: seq[T]): string =
-  # XXX why is that not in the stdlib?
   result = "["
   for i, x in s:
     if i > 0: result.add(", ")
@@ -59,7 +58,7 @@ type
 # inherited fields are ignored, so no fields are set
 when true:
   var
-    o: A
+    o: B
   o = B(x: 123)
   echo o
   o = B(y: 678, x: 123)