What steps will reproduce the problem?
1. file: "main.go" :

package main
import "fmt";

type doer interface { do(); }
type Doer struct { do func(); }

func main()
{
  d := Doer{ func(){ fmt.Println("Done"); } };
  _ = doer(d);
}

2. compile with command: $ 8g main.go

What is the expected output? What do you see instead?
compile fails with output "Segmentation fault"

expected: either conversion is valid or isn't (preferably is ;) )

What is your $GOOS?  $GOARCH?
$GOOS : linux
$GOARCH : 386

Which revision are you using?  (hg identify)
4d7f5eddd695 tip
(was also present in release)

Please provide any additional information below.
For:
var d struct { do func(); };
d.do = func(){
  fmt.Println("Done");
};  
_ = doer(d);

compiler returns: 
"struct { do func() } is not doer
        missing do()"

Attachments:

1. main.go (216 bytes)
2. main1.go (238 bytes)