fix: correct index for embedded binary method receiver

When searching for a binary method on structures, look up on embedded
fields first, otherwise the resulting index is incorrect, as
reflect.Type.MethodByName succeeds also on container struct.

Fixes #834.

_test/cli6.go

@@ -0,0 +1,52 @@
+package main
+
+import (
+	"fmt"
+	"io/ioutil"
+	"log"
+	"net/http"
+	"net/http/httptest"
+)
+
+type T struct {
+	http.ResponseWriter
+}
+
+type mw1 struct {
+	next http.Handler
+}
+
+func (m *mw1) ServeHTTP(rw http.ResponseWriter, rq *http.Request) {
+	t := &T{
+		ResponseWriter: rw,
+	}
+	x := t.Header()
+	fmt.Fprint(rw, "Welcome to my website!", x)
+}
+
+func main() {
+	m1 := &mw1{}
+
+	mux := http.NewServeMux()
+	mux.HandleFunc("/", m1.ServeHTTP)
+
+	server := httptest.NewServer(mux)
+	defer server.Close()
+
+	client(server.URL)
+}
+
+func client(uri string) {
+	resp, err := http.Get(uri)
+	if err != nil {
+		log.Fatal(err)
+	}
+	body, err := ioutil.ReadAll(resp.Body)
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Println(string(body))
+}
+
+// Output:
+// Welcome to my website!map[]

interp/type.go

@@ -1244,21 +1244,19 @@ func (t *itype) lookupBinMethod(name string) (m reflect.Method, index []int, isP
 	if t.cat == ptrT {
 		return t.val.lookupBinMethod(name)
 	}
+	for i, f := range t.field {
+		if f.embed {
+			if m2, index2, isPtr2, ok2 := f.typ.lookupBinMethod(name); ok2 {
+				index = append([]int{i}, index2...)
+				return m2, index, isPtr2, ok2
+			}
+		}
+	}
 	m, ok = t.TypeOf().MethodByName(name)
 	if !ok {
 		m, ok = reflect.PtrTo(t.TypeOf()).MethodByName(name)
 		isPtr = ok
 	}
-	if !ok {
-		for i, f := range t.field {
-			if f.embed {
-				if m2, index2, isPtr2, ok2 := f.typ.lookupBinMethod(name); ok2 {
-					index = append([]int{i}, index2...)
-					return m2, index, isPtr2, ok2
-				}
-			}
-		}
-	}
 	return m, index, isPtr, ok
 }