Why is it permissible to set the <nil> type of interface in Go?

Question

http://play.golang.org/p/_GP3RZTh4Q

package main

import "fmt"

type TesterInterface interface{
    Yell()
}

type Tester struct{}

func (t Tester) Yell() {
    fmt.Println("HELLO")
}

func main() {
    var t TesterInterface
    t = Tester{}
    t.Yell()
    t = nil
    t.Yell()
}

I would expect the compiler to complain about line 19 ( t = nil), since it nildoes not Yell(), however, the program starts and provides the following output:

HELLO
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0xffffffff addr=0x0 pc=0x201a9]

goroutine 1 [running]:
runtime.panic(0xef060, 0x1b3d44)
    /tmp/sandbox/go/src/pkg/runtime/panic.c:266 +0xe0
runtime.panicstring(0x1b3d44, 0x1b85)
    /tmp/sandbox/go/src/pkg/runtime/panic.c:489 +0x120
runtime.sigpanic()
    /tmp/sandbox/go/src/pkg/runtime/os_nacl.c:254 +0x80
main.main()
    /tmpfs/gosandbox-229cdcdf_329829af_705be907_f12cf4dc_ed51e32e/prog.go:20 +0xa9
runtime.main()
    /tmp/sandbox/go/src/pkg/runtime/proc.c:220 +0x1c0
runtime.goexit()
    /tmp/sandbox/go/src/pkg/runtime/proc.c:1394

goroutine 2 [syscall]:
runtime.notetsleepg(0xfeefdf88, 0x0, 0xf8475800, 0xd)
    /tmp/sandbox/go/src/pkg/runtime/lock_sema.c:254 +0xa0
runtime.MHeap_Scavenger()
    /tmp/sandbox/go/src/pkg/runtime/mheap.c:463 +0xc0
runtime.goexit()
    /tmp/sandbox/go/src/pkg/runtime/proc.c:1394
created by runtime.main
    /tmp/sandbox/go/src/pkg/runtime/proc.c:179
 [process exited with non-zero status]

Program exited.

Why nildoes the compiler consider it legal to assign to an interface?

Also from Go-docs:

An interface type variable can hold a value of any type with a set of methods, which is any superset of an interface.

How does "nil" contain a set of methods, which is a superset TesterInterface? The docs say that

The value of an uninitialized interface type variable is nil.

But this seems to contradict the first statement.

Answer 1

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}

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---

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Answer 2

Go

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