I am creating a simple function in Scala

def addOne(m: Int): Int = m + 1

Using it with integers works normally, using double it throws a type mismatch error.

addOne(2) = 3
addOne(2.1) = error: type mismatch

When I use it with a character in double quotes, it throws a type mismatch as expected.

addOne("z") = error: type mismatch

However, when using a single quotes character it returns a value for that letter.

addOne('z') = 123

What is happening here and why is it like this?