Change branch of an object variant is allowed

[derekdai]

In section Object variants of manual, the example says

# invalid: would change the active object branch:
n.kind = nkInt

But in fact compiler accept it, and =destroy is called to prevent resources leak.

Example

type
  E = enum
    a, b, c, d
  X = object
    v: int
  O = object
    case kind: E
    of a:
      a: int
    of {b, c}:
      b: float
    else:
      d: X

proc `=destroy`(x: var X) =
  echo "x destroyed"

var o = O(kind: d, d: X(v: 12345))
echo o.d

o.kind = a
echo o.a

Current Output

$ nim r --gc:arc test.nim
(v: 12345)
x destroyed
0

=destroy will be called event with refc.

Expected Output

Compile with error.

Possible Solution

Additional Information

If I change d: X to d: ref X, with arc or orc, output

$ nim r --gc:arc test.nim
(v: 12345)
x destroyed
139738855620680

With refc

(v: 12345)
test.nim(22) test
~/.choosenim/toolchains/nim-#devel/lib/system/assign.nim(294) FieldDiscriminantCheck
~/.choosenim/toolchains/nim-#devel/lib/system/fatal.nim(53) sysFatal
Error: unhandled exception: assignment to discriminant changes object branch [FieldDefect]

Tested with 1.4.8 and devel

Nim Compiler Version 1.5.1 [Linux: amd64]
Compiled at 2021-10-07
Copyright (c) 2006-2021 by Andreas Rumpf

git hash: f03872d99ec2d25de829415a75162f9d8297bb19
active boot switches: -d:release

This is expected - newruntime (arc and orc) do support changing active branch at runtime as long as you don't have a custom destructor attached to your object variant

[Araq]

I think this should continue to be disallowed, the workaround is o = O(kind: a, ...) (reassign the complete structure).

[ringabout]

I think this should continue to be disallowed

I agree. It should be covered by {.cast(uncheckedAssign).} instead.