What steps will reproduce the problem?
1. go get code.google.com/p/go-tour
2. go vet code.google.com/p/go-tour/prog

What is the expected output?

No panic.

What do you see instead?

INTERNAL PANIC: runtime error: invalid memory address or nil pointer dereference
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
    panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x38 pc=0x4eb38c]

goroutine 1 [running]:
code.google.com/p/go.tools/go/types.func·003()
    /home/u/goget/src/code.google.com/p/go.tools/go/types/check.go:113 +0x16a
code.google.com/p/go.tools/go/types.identicalMethods(0xc2101ed380, 0xc2100cffc0,
0xc2100e0310)
    /home/u/goget/src/code.google.com/p/go.tools/go/types/predicates.go:235 +0x28c
code.google.com/p/go.tools/go/types.IsIdentical(0xc21004f200, 0xc2101bdee8,
0xc21004f200, 0xc2100e0310, 0xc21005a800, ...)
    /home/u/goget/src/code.google.com/p/go.tools/go/types/predicates.go:170 +0x974
code.google.com/p/go.tools/go/types.(*operand).isAssignable(0xc2102138c0, 0xc2100d2480,
0xc21004f240, 0xc2100cff60, 0x41e500, ...)
    /home/u/goget/src/code.google.com/p/go.tools/go/types/operand.go:144 +0x172
code.google.com/p/go.tools/go/types.(*checker).assignment(0xc21005b900, 0xc2102138c0,
0xc21004f240, 0xc2100cff60, 0xc21004f240, ...)
    /home/u/goget/src/code.google.com/p/go.tools/go/types/stmt.go:36 +0x1dd
code.google.com/p/go.tools/go/types.(*checker).argument(0xc21005b900, 0xc2100d0600, 0x0,
0xc21005a740, 0xc210098560, ...)
    /home/u/goget/src/code.google.com/p/go.tools/go/types/expr.go:987 +0x23a
code.google.com/p/go.tools/go/types.(*checker).rawExpr(0xc21005b900, 0xc2102138c0,
0xc21005a900, 0xc21009c080, 0x0, ...)
    /home/u/goget/src/code.google.com/p/go.tools/go/types/expr.go:1603 +0x3c3b
code.google.com/p/go.tools/go/types.(*checker).stmt(0xc21005b900, 0xc21005aa00,
0xc210089d60)
    /home/u/goget/src/code.google.com/p/go.tools/go/types/stmt.go:356 +0x545c
code.google.com/p/go.tools/go/types.(*checker).stmtList(0xc21005b900, 0xc21009c0c0, 0x3,
0x4)
    /home/u/goget/src/code.google.com/p/go.tools/go/types/stmt.go:266 +0x68
code.google.com/p/go.tools/go/types.check(0xc2100d2480, 0x601020, 0x1, 0xc21005a540,
0xc2100ce800, ...)
    /home/u/goget/src/code.google.com/p/go.tools/go/types/check.go:159 +0x431
code.google.com/p/go.tools/go/types.(*Context).Check(0xc2100d2480, 0x601020, 0x1,
0xc21005a540, 0xc2100ce800, ...)
    /home/u/goget/src/code.google.com/p/go.tools/go/types/api.go:142 +0x6c
main.(*Package).check(0xc2100d23c0, 0xc21005a540, 0xc2100ce800, 0x42, 0x80, ...)
    /home/u/goget/src/code.google.com/p/go.tools/cmd/vet/types.go:32 +0x168
main.doPackage(0x601020, 0x1, 0xc21000a010, 0x42, 0x42, ...)
    /home/u/goget/src/code.google.com/p/go.tools/cmd/vet/main.go:226 +0x8f5
main.main()
    /home/u/goget/src/code.google.com/p/go.tools/cmd/vet/main.go:144 +0x665

goroutine 2 [runnable]:
exit status 2

Which compiler are you using (5g, 6g, 8g, gccgo)?
6g

Which operating system are you using?
Linux

Which version are you using?  (run 'go version')
go version devel +4aa7943034c5 Thu Jun 06 18:10:42 2013 -0700 linux/amd64

code.google.com/p/go.tools
changeset: 956fb611bffb

Please provide any additional information below.

The file that causes this panic is:
code.google.com/p/go-tour/prog/interfaces-are-satisfied-implicitly.go

Below is a simplified version that still causes the panic. I'll inline the
example since play.golang.org is almost impossible to access from some parts of
Asia.

package main

import (
    "fmt"
    "os"
)

type Writer interface {
    Write(b []byte) (n int, err error)
}

func main() {
    var w Writer

    // os.Stdout implements Writer
    w = os.Stdout

    fmt.Fprintf(w, "hello, writer\n")
}