Struct not recognized as `http.Flusher` after conversion to `http.ResponseWriter` interface

[okkero]

The following program sample.go triggers an unexpected result

package main

import (
	"fmt"
	"net/http"
	"reflect"
)

type responseWriter struct {
	http.ResponseWriter
}

func (w *responseWriter) Flush() {
	fmt.Println("Flushing...")
	if flusher, ok := w.ResponseWriter.(http.Flusher); ok {
		flusher.Flush()
	}
}

type dummy struct{}

func (d *dummy) Header() http.Header {
	fmt.Println("Dummy Header")
	return nil
}

func (d *dummy) Write(data []byte) (int, error) {
	fmt.Println("Dummy Write ", data)
	return 0, nil
}

func (d *dummy) WriteHeader(responseCode int) {
	fmt.Println("Dummy WriteHeader ", responseCode)
}

func foo(rw http.ResponseWriter) {
	var respCodeOverrideWriter = &responseWriter{ResponseWriter: rw}

	var flusher http.Flusher = respCodeOverrideWriter
	fmt.Println("responseWriter is http.Flusher. We can call Flush manually:")
	flusher.Flush()

	var httpResponseWriter http.ResponseWriter = respCodeOverrideWriter
	_, ok := httpResponseWriter.(http.Flusher)
	fmt.Println("Is responseWriter http.Flusher?", ok)
}

func main() {
	foo(&dummy{})
}

Expected result

responseWriter is http.Flusher. We can call Flush manually:
Flushing...
Is responseWriter http.Flusher? true

Got

responseWriter is http.Flusher. We can call Flush manually:
Flushing...
Is responseWriter http.Flusher? false

Yaegi Version

v0.15.1

Additional Notes

See this issue in the Traefik repo for context