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

package main

import "bufio"

func (x ***int) m() {}
func (x chan int) m() {}
func (x *bufio.Reader) m() {}

The compiler can print good errors for this (click compile):

prog.go:5: invalid receiver type ***int (**int is an unnamed type)
prog.go:6: invalid receiver type chan int (chan int is an unnamed type)
prog.go:7: cannot define new methods on non-local type bufio.Reader

go/parser rejects it early (click format):

/tmp/x.go:5:10: expected (unqualified) identifier
/tmp/x.go:6:9: expected (unqualified) identifier
/tmp/x.go:7:10: expected (unqualified) identifier

Since ast.FuncDecl.Recv is just a *FieldList, it seems like the parser can easily accept
and represent the input without changes to go/ast or go/printer, and then go/types (or a
compiler using go/parser) can do the semantic rejection of the receiver types at a
higher level and give better errors.