Donald Trump undergoes a medical assessment every year carried out by certified professionals. These have not revealed any mental health issues.
In 2018, Ronny Jackson asserted that Donald Trump was in "excellent" physical and mental condition. Then in 2019, a group of 11 medical specialists concluded Trump was in "very good health overall." The cognitive tests are not usually included in the president's annual physical. However, in response to rumours circulating about his mental health, Trump took a cognitive assessment (the Montreal Cognitive Assessment) and scored a perfect score.
Even if the medical had uncovered a serious mental health issue, the public would not know about it. The president is granted the same rights to health privacy as everyone else. This means that he is allowed to decide what details about his health are released to the public. In his assessment, medical professionals could have diagnosed him with Alzheimer's or dementia, but the public would have no right to that information. In fact, it is almost certain that any mental illness would not be disclosed out of national security concerns.
[P1] Trump has taken both mental and physical health assessments. [P2] Both revealed the president is in perfect health. [P3] Therefore, he is not mentally ill.
Rejecting the premises
[Rejecting P2] Even if the assessments revealed mental illness, the public reports would not include it.