Some spells are melee attacks (PHB 195) [whoo chill touch!]
Those can be used when melee attacks are required – such as the Mobile feat: thus chill touch counts as a melee attack that doesn’t provoke opportunity attacks, if you have the Mobile Feat! Booyah!
Some other melee spell attacks:
- shocking grasp
- thorn whip
- vampiric touch
- flame blade