Broken assignment of union with bool inside variant object

[bptato]

Nim Version

Nim Compiler Version 2.3.1 [Linux: amd64]
Compiled at 2025-10-20
Copyright (c) 2006-2025 by Andreas Rumpf

git hash: 1eae14a
active boot switches: -d:release

Description

Following code asserts with clang 19:

type
  A* {.union.} = object
    c*: C
    b*: bool

  B* = object
    case x*: bool
    of false:
      a*: A
    of true:
      cvar*: RootRef

  C* = enum
    wrong1, wrong2, right

proc main() =
  var xs: seq[B]
  let a = B(x: false, a: A(c: right))
  var b = a
  xs.add(a)
  assert b.a.c == right

main()

Looking at the generated code, I see:

N_LIB_PRIVATE N_NIMCALL(void, eqcopy___x_u169)(tyObject_B__X9cjIG3K6Mr4MzNE0Eyi6Uw* dest_p0, tyObject_B__X9cjIG3K6Mr4MzNE0Eyi6Uw src_p1) {
	tyObject_B__X9cjIG3K6Mr4MzNE0Eyi6Uw colontmp_;
nimZeroMem(((void*) ((&colontmp_))), sizeof(tyObject_B__X9cjIG3K6Mr4MzNE0Eyi6Uw));
colontmp_ = (*dest_p0);
nimZeroMem(((void*) ((&(*dest_p0)))), sizeof(tyObject_B__X9cjIG3K6Mr4MzNE0Eyi6Uw));
(*dest_p0).x = src_p1.x;
switch ((*dest_p0).x) {
case NIM_FALSE:
	{
		nimZeroMem(((void*) ((&(*dest_p0)._x_1.a))), sizeof(tyObject_A__nYOkuL9cKb3K8tmh8t0Ft9aA));
(*dest_p0)._x_1.a.c = src_p1._x_1.a.c;
(*dest_p0)._x_1.a.b = src_p1._x_1.a.b;
}
	break;

Aside from the obvious redundancy of assigning each field in the union, C's bool is not specified to be capable of representing all 8-bit values (just 0 and 1), so reading it here is UB. In practice the code happens to work in GCC, but clang happily zeroes out the upper bytes, flipping c from right to wrong1.

Current Output

Error: unhandled exception: /tmp/x.nim(29, 3) `b.a.c == right`  [AssertionDefect]

Expected Output

no assertion

Known Workarounds

proc `=copy`(a: var A; b: A) =
  copyMem(addr a, unsafeAddr b, sizeof(A))

proc `=destroy`(a: var A) = # seems to be needed too for sink
  discard

.union may not contain GC'ed types, so I guess the compiler could generate the same.

Additional Information

No response