How does custom scalar directives work?

[karlc1]

I cannot get a custom scalar directive to fire, am I misunderstanding the usage of them or is there something wrong with gqlgen? If I do something like this:

directive @onScalar on SCALAR
scalar MyScalar @onScalar

input MyInput {
  in: MyScalar
}

type mutation {
  doSomething(in: MyInput): Boolean
}

The directive function is never executed when I call the doSomething mutation. Is it not supposed to fire whenever the custom scalar is used?