Does a failed spellcasting because of Counterspell still count as a trigger for Mage Slayer’s attack?

@JeremyECrawford Does a failed spellcasting because of Counterspell still count as a trigger for Mage Slayer's attack?

— Dimas Pradipta (@dimas_pr) September 27, 2016

Counterspell causes a spell to fail. It doesn't undo the fact that spellcasting occurred. #DnD https://t.co/61gIfhmDkI

— Jeremy Crawford (@JeremyECrawford) September 27, 2016