io: remove SizedReaderAt

It's not clear we want to enshrine an io interface in which Size cannot
return an error. Because this requires more thought before committing
to the API, remove from Go 1.7.

Fixes #15818.

Change-Id: Ic4138ffb0e033030145a12d33f78078350a8381f
Reviewed-on: https://go-review.googlesource.com/23392Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>

api/next.txt

@@ -167,9 +167,6 @@ pkg io, const SeekEnd = 2
 pkg io, const SeekEnd ideal-int
 pkg io, const SeekStart = 0
 pkg io, const SeekStart ideal-int
-pkg io, type SizedReaderAt interface { ReadAt, Size }
-pkg io, type SizedReaderAt interface, ReadAt([]uint8, int64) (int, error)
-pkg io, type SizedReaderAt interface, Size() int64
 pkg math/big, method (*Float) GobDecode([]uint8) error
 pkg math/big, method (*Float) GobEncode() ([]uint8, error)
 pkg net, method (*Dialer) DialContext(context.Context, string, string) (Conn, error)

src/io/io.go

@@ -274,16 +274,6 @@ type RuneScanner interface {
 	UnreadRune() error
 }
 
-// SizedReaderAt is the interface that groups the basic ReadAt method
-// with a Size method that reports the total size of the underlying
-// object. It represents a fixed-size data source that supports random
-// access by multiple concurrent goroutines.
-type SizedReaderAt interface {
-	ReaderAt
-	// Size reports the length of the data source in bytes.
-	Size() int64
-}
-
 // stringWriter is the interface that wraps the WriteString method.
 type stringWriter interface {
 	WriteString(s string) (n int, err error)