interp: fix type check of methods with a receiver of interface kind

Fixes #1280.

_test/issue-1280.go

@@ -0,0 +1,30 @@
+package main
+
+import (
+	"io"
+	"log"
+	"os"
+)
+
+type DBReader interface {
+	io.ReadCloser
+	io.ReaderAt
+}
+
+type DB struct {
+	f DBReader
+}
+
+func main() {
+	f, err := os.Open("/dev/null")
+	if err != nil {
+		log.Fatal(err)
+	}
+	d := &DB{f}
+	data := make([]byte, 1)
+	_, _ = d.f.ReadAt(data, 0)
+	println("bye")
+}
+
+// Output:
+// bye

interp/type.go

@@ -1071,7 +1071,7 @@ func (t *itype) in(i int) *itype {
 		return t.arg[i]
 	case valueT:
 		if t.rtype.Kind() == reflect.Func {
-			if t.recv != nil {
+			if t.recv != nil && !isInterface(t.recv) {
 				i++
 			}
 			if t.rtype.IsVariadic() && i == t.rtype.NumIn()-1 {