From bug 1838703, here's the root cause analysis for the regression surfaced (though not introduced) by bug 1837557.

class OwningGPUOutOfMemoryErrorOrGPUValidationError : public AllOwningUnionBase
[...]
  union Value
  {
    UnionMember<OwningNonNull<mozilla::webgpu::OutOfMemoryError> > mGPUOutOfMemoryError;
    UnionMember<OwningNonNull<mozilla::webgpu::ValidationError> > mGPUValidationError;

  };

  TypeOrUninit mType;
  Value mValue;

OwningNonNull<mozilla::webgpu::OutOfMemoryError>&
OwningGPUOutOfMemoryErrorOrGPUValidationError::SetAsGPUOutOfMemoryError()
{
  if (mType == eGPUOutOfMemoryError) {
    return mValue.mGPUOutOfMemoryError.Value();
  }
  Uninit();
  mType = eGPUOutOfMemoryError;
  return mValue.mGPUOutOfMemoryError.SetValue();
}

class UnionMember {
  AlignedStorage2<T> mStorage;
[...]
  template <typename... Args>
  T& SetValue(Args&&... args) {
    new (mStorage.addr()) T(std::forward<Args>(args)...);
    return *mStorage.addr();
  }

template <class T>
class MOZ_IS_SMARTPTR_TO_REFCOUNTED OwningNonNull {
 public:
  using element_type = T;

  OwningNonNull() = default;

The problem is that SetValue() in-place constructs the union variant, and in this case the union variant isn't OutOfMemory (which has a deleted ctor), it's OwningNonNull<OutOfMemory>, which does have a super dangerous default constructor!

The patch here is required for the patch in bug 1838703 to work, since bug 1838703 catches this issue.

Set release status flags based on info from the regressing bug 1837557

Backed out for causing build bustages on OutOfMemoryError.h.

Push with failures: https://treeherder.mozilla.org/jobs?repo=autoland&resultStatus=testfailed%2Cbusted%2Cexception%2Cretry%2Cusercancel&revision=8599db8132f994309afbe27805208fb147fbcf0c&selectedTaskRun=fYFkA8OaROKPW40Gg575sQ.0

Failure log: https://treeherder.mozilla.org/logviewer?job_id=419505508&repo=autoland

Backout link: https://hg.mozilla.org/integration/autoland/rev/fa529bbf11c86465c2e1ecfd68b07ae27cbfcb87

I believe that :jgilbert has addressed the cause of the backout in D181161 already (added explicit to the OutOfMemoryError constructor), so I'm gonna try to land this.

Backed out for causing build bustages in OutOfMemoryError.h

• Backout link
• Push with failures
• Failure Log
• Failure line: builds/worker/workspace/obj-build/dist/include/mozilla/webgpu/OutOfMemoryError.h:24:12: error: missing return type for function 'sOutOfMemoryError'; did you mean the constructor name 'OutOfMemoryError'?

https://hg.mozilla.org/mozilla-central/rev/1f0ecff11fae

Set release status flags based on info from the regressing bug 1837557