Solid forms including 7-[[(1S)-1-[4-[(1S)-2-cyclopropyl-1-(4-prop-2-enoylpiperazin-1-yl)ethyl]phenyl]ethyl]amino]-1-ethyl-4H-pyrimido[4,5-d][1,3]oxazin-2-one and a dihydroxybenzoic acid selected from 2,5-dihydroxybenzoic acid, 2,4-dihydroxybenzoic acid, and 2,3-dihydroxybenzoic acid are provided, as well as pharmaceutical compositions including the solid forms, methods for using the solid forms in treating cancer patients, and methods of making the solid forms.