Can Booming Blade cantrip valid as an attack of opportunity?

@JeremyECrawford Is the melee attack that happens concurrently with the booming blade cantrip valid as an attack of opportunity?

— Jeffrey (@ofthemagi) March 30, 2016

You can't cast a spell as an opportunity attack, unless you have a feature like the War Caster feat. #DnD https://t.co/xyJXgx7mLp

— Jeremy Crawford (@JeremyECrawford) March 30, 2016