Invalid dynamic type after slicing sink assignment

[arnetheduck]

Description

type A {.inheritable.} = object
type B = object of A

type C = object
  a: A

var c: C

proc f(v: sink A) =
  var a: A = move(v)
  echo a of B
  c.a = move(a)
  echo c.a of B

f(B())

Nim Version

2.2.0

Current Output

true
true

Expected Output

false
false

Known Workarounds

No response

Additional Information

No response

[ringabout]

See also #7002;

refc gives ObjectAssignmentDefect for now

[arnetheduck]

One tricky bit is the following:

proc f(v: sink A) =
  echo v of B

f(B())

what should this print? ie when exactly does the slicing happen?

[forchid]

The memory is out of order.

type A {.inheritable.} = object
type B = object of A
    value: int

type C = object
  a: A
  
method print(a: A) {.base.} =
    echo "A: no value"
    
method print(b: B) =
    echo "B: value ", b.value

var c: C

proc f(v: sink A) =
  v.print
  var a: A = move(v)
  a.print
  echo a of B
  c.a = move(a)
  c.a.print
  echo c.a of B
  
proc f1(v: sink A) =
  v.print
  echo v of B
  
proc f2(v: A) =
  v.print
  echo v of B

f(B(value: 1))
f1(B(value: 1))
f2(B(value: 1))
var v: A = B(value: 1)
echo v of B
v.print

The test result

mem>objcast_test
B: value 1
B: value 140694538682368
true
B: value 0
true
B: value 1
true
B: value 1
true
true
B: value 140696177906336

Nim version
2.2.2

[ringabout]

For the record, this example needs to continue to work

type
  A {.inheritable.} = object
  B = object of A
    f: int

proc `=destroy`(x: var B) = discard


proc `$$`[T](x: sink T): string =
  "{}"

let a: ref A = new(B)
doAssert $$a[] == "{}"