$ cat x.go
package p

import (
    "strings"
    "testing"
    . "bufio"
)

type negativeReader int

func (*negativeReader) Read([]byte) (int, error) {panic(1)}

func TestNegativeRead(t *testing.T) {
    // should panic with a description pointing at the reader, not at itself.
    // (should NOT panic with slice index error, for example.)
    b := NewReader(new(negativeReader))
    defer func() {
        if err := recover(); err = nil {
            t.Fatal("read did not panic")
        } else if !strings.Contains(err.Error(), "reader returned negative count from Read") {
            t.Fatal("wrong panic: %v", err)
        }
    }()
    b.Read(make([]byte, 100))
}

$ gofmt x.go
x.go:17:8: expected function/method call
x.go:18:28: expected '{', found '='
x.go:19:33: missing ',' before newline in composite literal
x.go:20:5: expected ';', found 'else'
x.go:23:3: expected ';', found '('
x.go:25:3: expected '}', found 'EOF'

$ go tool 6g x.go
x.go:18: err = nil used as value
x.go:20: err.Error undefined (type interface {} has no field or method Error)

It would be nice if the parser could handle seeing an assignment where an expression was
expected a bit more gracefully. Ideally it would produce just the single line of output
that 6g does (at x.go:18).