Can you use a staff for spells and abilities that require a weapon?

@JeremyECrawford Can you use a staff (that can be used as a quarterstaff) for spells and abilities that require a weapon, such as Green Flame Blade, Shillelagh, and the Warlocks Pact Weapon?

— Simon Cwilich (@kingslime80) March 15, 2018

A quarterstaff is a weapon, as is a staff that can be used as a quarterstaff. See the Weapons table for a list of weapons, including the quarterstaff (PH, 149). #DnD https://t.co/ofcUaKLtN5

— Jeremy Crawford (@JeremyECrawford) March 16, 2018