It could be a beginner's question. Code is like,
type MYMAP map[int]int
func (o *MYMAP) dosth(){
//this will fail to compile
o[1]=2
}
error message: invalid operation: o[1] (index of type *MYMAP)
How to access the underlying type of MYMAP as map?
The problem isn't that it's an alias, it's that it's a pointer to a map.
Go will not automatically deference pointers for map or slice access the way it will for method calls. Replacing o[1]=2 with (*o)[1]=2 will work. Though you should consider why you're (effectively) using a pointer to a map. There can be good reasons to do this, but usually you don't need a pointer to a map since maps are "reference types", meaning that you don't need a pointer to them to see the side effects of mutating them across the program.
An easy fix could be done by getting rid of pointer, just change o *MYMAP to o MYMAP
type MYMAP map[int]int
func (o MYMAP) dosth(){
o[1]=2
}