For:

$ cat test.go
package main

import "io"

func main() {
    var y interface {
        Read()
    }
    _ = y.(io.Reader)
}

Neither compiler complains:

$ gccgo test.go
$ go tool 6g test.go

And according to the spec it is a valid program.

However, any concrete value assigned to y must implement Read(), and thus cannot
possibly implement io.Reader.Read. Thus, this type assertion will fail and we know this
at compile time.

The spec should require this check and compilers should implement it. This would be a
backward-incompatible language change, albeit with a very small chance of actually
breaking existing code.