from irs pub 969
Other employee health plans. An employee covered by an HDHP and a health FSA or an HRA that pays
or reimburses qualified medical expenses generally can’t make contributions to an HSA
summary from IRS reg 1.125-4
An FSA's coverage period ends either at the time the plan year ends for one's plan or at the time when one's coverage under that plan ends. An example of such an event is the loss of coverage due to a separation from the employer. (1.125-4(c)(iii)
