weird ObjectAssignmentDefect runtime errors on based object class assignment

[timotheecour]

Example 1

when true:
  type A = object of RootObj
    x: int
  type B = object of A
    y: int
  proc fn1(a: var A) =
    echo (a,) # bug: ObjectAssignmentDefect
  proc fn2(a: A) =
    echo a.x # ok
    echo $a # ok
    echo ($a,) # ok
    echo (a,) # BUG

  proc main =
    var b = B(x: 3, y:4)
    fn2(b) # crash in RT, no crash in VM
    # fn1(b) # ditto

  main()
  static: main()

Current Output

• works in CT
• works in js
• works in gc:arc
• crashes in RT with gc:refc

3
(x: 3)
("(x: 3)",)
((x: 3),)
Hint: 55209 lines; 0.497s; 75.797MiB peakmem; Debug build; proj: /Users/timothee/git_clone/nim/timn/tests/nim/all/t12117.nim; out: /Users/timothee/git_clone/nim/timn/build/nimcache/t12117 [SuccessX]
/Users/timothee/git_clone/nim/Nim_devel/compiler/main.nim(377, 15) compiler msg initiated here [MsgOrigin]
3
(x: 3)
("(x: 3)",)
/Users/timothee/git_clone/nim/timn/tests/nim/all/t12117.nim(66) t12117
/Users/timothee/git_clone/nim/timn/tests/nim/all/t12117.nim(63) main
/Users/timothee/git_clone/nim/timn/tests/nim/all/t12117.nim(59) fn2
/Users/timothee/git_clone/nim/Nim_devel/lib/system/assign.nim(143) genericAssign
/Users/timothee/git_clone/nim/Nim_devel/lib/system/assign.nim(129) genericAssignAux
/Users/timothee/git_clone/nim/Nim_devel/lib/system/fatal.nim(53) sysFatal
Error: unhandled exception: invalid object assignment [ObjectAssignmentDefect]

Expected Output

• VM, RT, js should have same behavior
• that behavior should be that it works
  (similar code works in C++)
• even if it wasn't supposed to work, it should give CT error, not RT crash

Example 2

when true:
  type A = object of RootObj
    x: int
  type B = object of A
    y: int

  proc main =
    var b: B
    var a: A
    a = b # VM or js: no crash, RT: ObjectAssignmentDefect

  static: main()
  main()

same behavior as Example 1

Additional Information

1.5.1 fe9a37f

links

• genericAssignAux runtime error #16706
• Broken method behavior with derived objects #7002 seems related but this issue has no method

similar code works in C++

#include <iostream>
#include <stdio.h>

struct A{int x;};
struct B: public A{int y;};
void fn(A a){
  printf("%d\n", a.x);
}
int main (int argc, char *argv[]) {
  A a;
  B b;
  b.x = 3;
  b.y = 4;
  a = b;
  fn(a);
  fn(b);
  return 0;
}

[ringabout]

You can use ref object.

[timotheecour]

it should work IMO (eg suppose i want stack allocation and don't want ref)

• similar code (without pointer) works in C++
• it works in js, CT, gc:arc

IMO it should work in gc:refc

[ringabout]

Related:
#11592
https://forum.nim-lang.org/t/7733