C++ Atomics: operator= is implicitly deleted because the default definition would be ill-formed

[mratsim]

Seems like C++ atomics don't like me at all.

When trying to workaround #13062

import atomics

type
  Pledge* = object
    p: PledgePtr

  PledgeKind = enum
    Single
    Iteration

  PledgePtr = ptr object
    refCount: Atomic[int32]
    impl: PledgeImpl
    # The case object is not important for the bug
    # but the non-binary 0 case requires the object constructor-based
    # initialization as we can't change the case object once constructed
    case kind: PledgeKind
    of Single:
      discard
    of Iteration:
      discard

  PledgeImpl = object
    fulfilled: Atomic[bool]

template deref*(T: typedesc): typedesc =
  ## Return the base object type behind a ptr type
  typeof(default(T)[])

proc main() =
  var pledge: Pledge
  pledge.p = createShared(deref(PledgePtr))
  zeroMem(pledge.p, sizeof(deref(PledgePtr)))

  # Bad codegen
  pledge.p[] = deref(PledgePtr)(kind: Iteration)

main()

Error

..../.cache/nim/atombug_d/@matombug.nim.cpp: In function ‘void main__b3r7EXSeAzsO9aIDCMEJ8CA()’:
..../.cache/nim/atombug_d/@matombug.nim.cpp:157:16: error: use of deleted function ‘tyObject_PledgePtrcolonObjectType___FkbYobdvoyzypSol0uBgbw& tyObject_PledgePtrcolonObjectType___FkbYobdvoyzypSol0uBgbw::operator=(const tyObject_PledgePtrcolonObjectType___FkbYobdvoyzypSol0uBgbw&)’
  157 |  (*pledge.p) = T1_;
      |                ^~~
..../.cache/nim/atombug_d/@matombug.nim.cpp:47:8: note: ‘tyObject_PledgePtrcolonObjectType___FkbYobdvoyzypSol0uBgbw& tyObject_PledgePtrcolonObjectType___FkbYobdvoyzypSol0uBgbw::operator=(const tyObject_PledgePtrcolonObjectType___FkbYobdvoyzypSol0uBgbw&)’ is implicitly deleted because the default definition would be ill-formed:
   47 | struct tyObject_PledgePtrcolonObjectType___FkbYobdvoyzypSol0uBgbw {
      |        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
..../.cache/nim/atombug_d/@matombug.nim.cpp:47:8: error: use of deleted function ‘std::atomic<int>& std::atomic<int>::operator=(const std::atomic<int>&)’
In file included from ..../.cache/nim/atombug_d/@matombug.nim.cpp:11:
/usr/include/c++/9.2.0/atomic:756:15: note: declared here
  756 |       atomic& operator=(const atomic&) = delete;
      |               ^~~~~~~~
..../.cache/nim/atombug_d/@matombug.nim.cpp:47:8: error: use of deleted function ‘tyObject_PledgeImpl__PC08ohoFN4aei4c6kCPnAw& tyObject_PledgeImpl__PC08ohoFN4aei4c6kCPnAw::operator=(const tyObject_PledgeImpl__PC08ohoFN4aei4c6kCPnAw&)’
   47 | struct tyObject_PledgePtrcolonObjectType___FkbYobdvoyzypSol0uBgbw {
      |        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
..../.cache/nim/atombug_d/@matombug.nim.cpp:43:8: note: ‘tyObject_PledgeImpl__PC08ohoFN4aei4c6kCPnAw& tyObject_PledgeImpl__PC08ohoFN4aei4c6kCPnAw::operator=(const tyObject_PledgeImpl__PC08ohoFN4aei4c6kCPnAw&)’ is implicitly deleted because the default definition would be ill-formed:
   43 | struct tyObject_PledgeImpl__PC08ohoFN4aei4c6kCPnAw {
      |        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
..../.cache/nim/atombug_d/@matombug.nim.cpp:43:8: error: use of deleted function ‘std::atomic<bool>& std::atomic<bool>::operator=(const std::atomic<bool>&)’
In file included from ..../.cache/nim/atombug_d/@matombug.nim.cpp:11:
/usr/include/c++/9.2.0/atomic:74:13: note: declared here
   74 |     atomic& operator=(const atomic&) = delete;
      |             ^~~~~~~~

[mratsim]

Even this very minimal example doesn't compile for some reason

import atomics

type
  Pledge* = object
    p: PledgePtr

  PledgePtr = ptr PledgeObj
  PledgeObj = object
    refCount: Atomic[int32]

proc main() =
  var pledge: Pledge
  pledge.p = createShared(PledgeObj)
  let tmp = PledgeObj()

  pledge.p[] = tmp

main()

Note that I have plenty of code with atomics that does compile in C++ mode

[mratsim]

To answer the small example, this is actually not allowed

import atomics

type
  Pledge* = object
    p: PledgePtr

  PledgePtr = ptr PledgeObj
  PledgeObj = object
    refCount: Atomic[int32]

proc main() =
  var pledge: Pledge
  pledge.p = createShared(PledgeObj)
  let tmp = PledgeObj() # <---- not allowed: atomics are not copyable

  pledge.p[] = tmp

main()

The code generated is

N_LIB_PRIVATE N_NIMCALL(void, main__BTA1O9bZ9cw3phtcEzYReOQw)(void) {
	tyObject_Pledge__liVKkHLjQBejXUzfhcSWAw pledge;
	tyObject_PledgeObj__JS8Q9a9bu0tSuejYr9a1kzS1A T1_;
	nimZeroMem((void*)(&pledge), sizeof(tyObject_Pledge__liVKkHLjQBejXUzfhcSWAw));
	pledge.p = createShared__w7DYWNtNZpqDzb9bgOkT5tAsystem(((NI) 1));
	nimZeroMem((void*)(&T1_), sizeof(tyObject_PledgeObj__JS8Q9a9bu0tSuejYr9a1kzS1A));
	tyObject_PledgeObj__JS8Q9a9bu0tSuejYr9a1kzS1A tmp = T1_;
	(*pledge.p) = tmp;
}

and tyObject_PledgeObj__JS8Q9a9bu0tSuejYr9a1kzS1A tmp = T1_; is not allowed due to the presence of atomics.

In the past, due to the use of case objects and the fact that we can't change case this is required for
pledge.p[] = deref(PledgePtr)(kind: Iteration)

but the case-object transition rules seem to have been relaxed so now this seems to be a valid workaround?

import atomics

type
  Pledge* = object
    p: PledgePtr

  PledgeKind = enum
    Single
    Iteration

  PledgePtr = ptr object
    refCount: Atomic[int32]
    impl: PledgeImpl
    # The case object is not important for the bug
    # but the non-binary 0 case requires the object constructor-based
    # initialization as we can't change the case object once constructed
    case kind: PledgeKind
    of Single:
      discard
    of Iteration:
      discard

  PledgeImpl = object
    fulfilled: Atomic[bool]

template deref*(T: typedesc): typedesc =
  ## Return the base object type behind a ptr type
  typeof(default(T)[])

proc main() =
  var pledge: Pledge
  pledge.p = createShared(deref(PledgePtr))
  zeroMem(pledge.p, sizeof(deref(PledgePtr)))

  # Case object transition avoids invalid copy/sink
  pledge.p.kind = Iteration

main()