Why does golang implement different behavior on `[]`operator between slice and map? [duplicate]

type S struct {
    e int
}

func main() {
    a := []S{{1}}
    a[0].e = 2

    b := map[int]S{0: {1}}
    b[0].e = 2 // error
}

a[0] is addressable but b[0] is not.

I know first 0 is an index and second 0 is a key.

Why golang implement like this? Any further consideration?

I've read source code of map in github.com/golang/go/src/runtime and map structure already supported indirectkey and indirectvalue if maxKeySize and maxValueSize are little enough.

type maptype struct {
    ...
    keysize       uint8  // size of key slot
    indirectkey   bool   // store ptr to key instead of key itself
    valuesize     uint8  // size of value slot
    indirectvalue bool   // store ptr to value instead of value itself
    ...
}

I think if golang designers want this syntax, it works easy now.

Of course indirectkey indirectvalue may cost more resource and GC also need do more work.

So performance is the only reason for supporting this?

Or any other consideration?

In my opinion, supporting syntax like this is valuable.