IndexOfAll · Software Engineering Wiki

Returns all indexes of a given element in the collection, or [-1] if it is not present.

Use reflect.ValueOf() to get the array or slice and the value to search for, make() to create an appropriate slice.
Use a for loop with Value.Len() and Value.Index() to iterate over each element and compare it to the search value.
Use append() to add indexes of matching elements to the resulting slice.

import "reflect"

func IndexOfAll(params ...interface{}) []int {
	arr, v, r := reflect.ValueOf(params[0]),
		reflect.ValueOf(params[1]).Interface(),
		make([]int, 0)

	for i := 0; i < arr.Len(); i++ {
		if arr.Index(i).Interface() == v {
			r = append(r, i)
		}
	}
	if len(r) != 0 {
		return r
	}
	return []int{-1}
}

IndexOfAll([]int{1, 2, 1, 3, 1, 1}, 1) // [0 2 4 5]
IndexOfAll([]int{1, 2, 1, 3, 1, 1}, 4) // [-1]