encoding/json: Unmarshal: slice handling underspecified?

[masiulaniec]

In the below code (from http://play.golang.org/p/gX3MRTQyPA), it appears impossible to
predict program's output based solely on documentation of package encoding/json.

    list := []int{-1}
    err := json.Unmarshal([]byte("[1]"), &list)
    if err != nil 
        log.Fatal(err)
    }
    log.Printf("%+v", list)

I can think of the following likely outcomes:

  [-1]
  [1]
  [-1, 1]

Package encoding/json chooses [1] but I cannot seem to find a sentence that guarantees
it.

In contrast, relevant quote from package encoding/xml: "Unmarshal maps an XML
element to a slice by extending the length of the slice and mapping the element to the
newly created value."

[ianlancetaylor]

Comment 1:

Labels changed: added repo-main, release-go1.5, documentation.