From my understanding, you still go ahead and file anyways, and have all your documentation ready to prove the child lives with you for at least 6 months and a day, but do not send in or submit this documentation with your return. The IRS will most likely reach out to inform someone else has claimed this dependent and will inquire about it, then you state your situation and they will most likely ask you to fax these documents of proof the child is your legal dependent under the federal law to claim for the tax year you are filing under. The rest should take care of itself as they will go after the claimant fraudulently filing for your dependent. At least that's how it was in 2017. I would ask a tax consultant, or even call the IRS and ask them.
