Even if the other dependency tests are met, you are not allowed an exemption for your dependent if he or she files a joint return. (Exemptions are no longer available starting with the 2018 tax year)
Joint Return Test
You generally can't claim a married person as a dependent if he or she files a joint return.
Example 1—child files joint return.
Example 2—child files joint return only as claim for refund of withheld tax.
Example 3—child files joint return to claim American opportunity credit.