This is a fairly common occurrence in the endgame. However, White has a shortage of liberties.
What now?[1]
Anytime you find yourself about to make a heavy shape such as the farmer's hat above...it is best to take a second look because you are fairly certain to be somewhat (!) short on liberties. If black tries to peep now, a and b are miai for life.
[1]
SnotNose: kills. White cannot play c (shortage of liberties). If White b, Black c and the eye at d becomes false.
Bill: If Bc instead, lives.