What steps will reproduce the problem?
1. Run go.exp/gotype on

package main

import (
    "io"
)

func main() {
    var x io.Reader
    _, ok := x.(*io.MultiRider)
    println(ok)
}

What is the expected output? What do you see instead?

Expected:
something saying io.Multirider does not exist

Got:
test.go:9:15: cannot refer to unexported io.MultiRider
(io.Multirider seems perfectly exported to me!)

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