For a function, it makes sense to just write two functions, the easiest to use, and the one with maximum flaxibility.

    func SortOrdered[T constraints.Ordered](s []T) {
      // …
    }

    func Sort[T any](s []T, less func(T, T) bool) {
      // …
    }

For a data structure, I recommend using

    type SearchTree[T any] struct {
      Less func(T, T) bool
      // …
    }

and then writing two constructors

    func NewSearchTree(less func(T, T) bool) *SearchTree[T] {
      // …
      return &SearchTree[T]{
        Less: less,
        // …
      }
    }

    func NewOrderedSearchTree[T constraints.Ordered]() *SearchTree[T] {
      // …
      return &SearchTree[T]{
        Less: func(x, y T) bool {return x < y},
        // …
      }
    }