diff --git a/prometheus/promhttp/delegator.go b/prometheus/promhttp/delegator.go
new file mode 100644
index 0000000..5ee095b
--- /dev/null
+++ b/prometheus/promhttp/delegator.go
@@ -0,0 +1,199 @@
+// Copyright 2017 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package promhttp
+
+import (
+	"bufio"
+	"io"
+	"net"
+	"net/http"
+)
+
+const (
+	closeNotifier = 1 << iota
+	flusher
+	hijacker
+	readerFrom
+	pusher
+)
+
+type delegator interface {
+	http.ResponseWriter
+
+	Status() int
+	Written() int64
+}
+
+type responseWriterDelegator struct {
+	http.ResponseWriter
+
+	handler, method    string
+	status             int
+	written            int64
+	wroteHeader        bool
+	observeWriteHeader func(int)
+}
+
+func (r *responseWriterDelegator) Status() int {
+	return r.status
+}
+
+func (r *responseWriterDelegator) Written() int64 {
+	return r.written
+}
+
+func (r *responseWriterDelegator) WriteHeader(code int) {
+	r.status = code
+	r.wroteHeader = true
+	r.ResponseWriter.WriteHeader(code)
+	if r.observeWriteHeader != nil {
+		r.observeWriteHeader(code)
+	}
+}
+
+func (r *responseWriterDelegator) Write(b []byte) (int, error) {
+	if !r.wroteHeader {
+		r.WriteHeader(http.StatusOK)
+	}
+	n, err := r.ResponseWriter.Write(b)
+	r.written += int64(n)
+	return n, err
+}
+
+type closeNotifierDelegator struct{ *responseWriterDelegator }
+type flusherDelegator struct{ *responseWriterDelegator }
+type hijackerDelegator struct{ *responseWriterDelegator }
+type readerFromDelegator struct{ *responseWriterDelegator }
+
+func (d *closeNotifierDelegator) CloseNotify() <-chan bool {
+	return d.ResponseWriter.(http.CloseNotifier).CloseNotify()
+}
+func (d *flusherDelegator) Flush() {
+	d.ResponseWriter.(http.Flusher).Flush()
+}
+func (d *hijackerDelegator) Hijack() (net.Conn, *bufio.ReadWriter, error) {
+	return d.ResponseWriter.(http.Hijacker).Hijack()
+}
+func (d *readerFromDelegator) ReadFrom(re io.Reader) (int64, error) {
+	if !d.wroteHeader {
+		d.WriteHeader(http.StatusOK)
+	}
+	n, err := d.ResponseWriter.(io.ReaderFrom).ReadFrom(re)
+	d.written += n
+	return n, err
+}
+
+var pickDelegator = make([]func(*responseWriterDelegator) delegator, 32)
+
+func init() {
+	// TODO(beorn7): Code generation would help here.
+	pickDelegator[0] = func(d *responseWriterDelegator) delegator { // 0
+		return d
+	}
+	pickDelegator[closeNotifier] = func(d *responseWriterDelegator) delegator { // 1
+		return closeNotifierDelegator{d}
+	}
+	pickDelegator[flusher] = func(d *responseWriterDelegator) delegator { // 2
+		return flusherDelegator{d}
+	}
+	pickDelegator[flusher+closeNotifier] = func(d *responseWriterDelegator) delegator { // 3
+		return struct {
+			*responseWriterDelegator
+			http.Flusher
+			http.CloseNotifier
+		}{d, &flusherDelegator{d}, &closeNotifierDelegator{d}}
+	}
+	pickDelegator[hijacker] = func(d *responseWriterDelegator) delegator { // 4
+		return hijackerDelegator{d}
+	}
+	pickDelegator[hijacker+closeNotifier] = func(d *responseWriterDelegator) delegator { // 5
+		return struct {
+			*responseWriterDelegator
+			http.Hijacker
+			http.CloseNotifier
+		}{d, &hijackerDelegator{d}, &closeNotifierDelegator{d}}
+	}
+	pickDelegator[hijacker+flusher] = func(d *responseWriterDelegator) delegator { // 6
+		return struct {
+			*responseWriterDelegator
+			http.Hijacker
+			http.Flusher
+		}{d, &hijackerDelegator{d}, &flusherDelegator{d}}
+	}
+	pickDelegator[hijacker+flusher+closeNotifier] = func(d *responseWriterDelegator) delegator { // 7
+		return struct {
+			*responseWriterDelegator
+			http.Hijacker
+			http.Flusher
+			http.CloseNotifier
+		}{d, &hijackerDelegator{d}, &flusherDelegator{d}, &closeNotifierDelegator{d}}
+	}
+	pickDelegator[readerFrom] = func(d *responseWriterDelegator) delegator { // 8
+		return readerFromDelegator{d}
+	}
+	pickDelegator[readerFrom+closeNotifier] = func(d *responseWriterDelegator) delegator { // 9
+		return struct {
+			*responseWriterDelegator
+			io.ReaderFrom
+			http.CloseNotifier
+		}{d, &readerFromDelegator{d}, &closeNotifierDelegator{d}}
+	}
+	pickDelegator[readerFrom+flusher] = func(d *responseWriterDelegator) delegator { // 10
+		return struct {
+			*responseWriterDelegator
+			io.ReaderFrom
+			http.Flusher
+		}{d, &readerFromDelegator{d}, &flusherDelegator{d}}
+	}
+	pickDelegator[readerFrom+flusher+closeNotifier] = func(d *responseWriterDelegator) delegator { // 11
+		return struct {
+			*responseWriterDelegator
+			io.ReaderFrom
+			http.Flusher
+			http.CloseNotifier
+		}{d, &readerFromDelegator{d}, &flusherDelegator{d}, &closeNotifierDelegator{d}}
+	}
+	pickDelegator[readerFrom+hijacker] = func(d *responseWriterDelegator) delegator { // 12
+		return struct {
+			*responseWriterDelegator
+			io.ReaderFrom
+			http.Hijacker
+		}{d, &readerFromDelegator{d}, &hijackerDelegator{d}}
+	}
+	pickDelegator[readerFrom+hijacker+closeNotifier] = func(d *responseWriterDelegator) delegator { // 13
+		return struct {
+			*responseWriterDelegator
+			io.ReaderFrom
+			http.Hijacker
+			http.CloseNotifier
+		}{d, &readerFromDelegator{d}, &hijackerDelegator{d}, &closeNotifierDelegator{d}}
+	}
+	pickDelegator[readerFrom+hijacker+flusher] = func(d *responseWriterDelegator) delegator { // 14
+		return struct {
+			*responseWriterDelegator
+			io.ReaderFrom
+			http.Hijacker
+			http.Flusher
+		}{d, &readerFromDelegator{d}, &hijackerDelegator{d}, &flusherDelegator{d}}
+	}
+	pickDelegator[readerFrom+hijacker+flusher+closeNotifier] = func(d *responseWriterDelegator) delegator { // 15
+		return struct {
+			*responseWriterDelegator
+			io.ReaderFrom
+			http.Hijacker
+			http.Flusher
+			http.CloseNotifier
+		}{d, &readerFromDelegator{d}, &hijackerDelegator{d}, &flusherDelegator{d}, &closeNotifierDelegator{d}}
+	}
+}
diff --git a/prometheus/promhttp/delegator_1_8.go b/prometheus/promhttp/delegator_1_8.go
index b7743fb..f4d386f 100644
--- a/prometheus/promhttp/delegator_1_8.go
+++ b/prometheus/promhttp/delegator_1_8.go
@@ -20,54 +20,162 @@ import (
 	"net/http"
 )
 
-// newDelegator handles the four different methods of upgrading a
-// http.ResponseWriter to delegator.
+type pusherDelegator struct{ *responseWriterDelegator }
+
+func (d *pusherDelegator) Push(target string, opts *http.PushOptions) error {
+	return d.ResponseWriter.(http.Pusher).Push(target, opts)
+}
+
+func init() {
+	pickDelegator[pusher] = func(d *responseWriterDelegator) delegator { // 16
+		return pusherDelegator{d}
+	}
+	pickDelegator[pusher+closeNotifier] = func(d *responseWriterDelegator) delegator { // 17
+		return struct {
+			*responseWriterDelegator
+			http.Pusher
+			http.CloseNotifier
+		}{d, &pusherDelegator{d}, &closeNotifierDelegator{d}}
+	}
+	pickDelegator[pusher+flusher] = func(d *responseWriterDelegator) delegator { // 18
+		return struct {
+			*responseWriterDelegator
+			http.Pusher
+			http.Flusher
+		}{d, &pusherDelegator{d}, &flusherDelegator{d}}
+	}
+	pickDelegator[pusher+flusher+closeNotifier] = func(d *responseWriterDelegator) delegator { // 19
+		return struct {
+			*responseWriterDelegator
+			http.Pusher
+			http.Flusher
+			http.CloseNotifier
+		}{d, &pusherDelegator{d}, &flusherDelegator{d}, &closeNotifierDelegator{d}}
+	}
+	pickDelegator[pusher+hijacker] = func(d *responseWriterDelegator) delegator { // 20
+		return struct {
+			*responseWriterDelegator
+			http.Pusher
+			http.Hijacker
+		}{d, &pusherDelegator{d}, &hijackerDelegator{d}}
+	}
+	pickDelegator[pusher+hijacker+closeNotifier] = func(d *responseWriterDelegator) delegator { // 21
+		return struct {
+			*responseWriterDelegator
+			http.Pusher
+			http.Hijacker
+			http.CloseNotifier
+		}{d, &pusherDelegator{d}, &hijackerDelegator{d}, &closeNotifierDelegator{d}}
+	}
+	pickDelegator[pusher+hijacker+flusher] = func(d *responseWriterDelegator) delegator { // 22
+		return struct {
+			*responseWriterDelegator
+			http.Pusher
+			http.Hijacker
+			http.Flusher
+		}{d, &pusherDelegator{d}, &hijackerDelegator{d}, &flusherDelegator{d}}
+	}
+	pickDelegator[pusher+hijacker+flusher+closeNotifier] = func(d *responseWriterDelegator) delegator { //23
+		return struct {
+			*responseWriterDelegator
+			http.Pusher
+			http.Hijacker
+			http.Flusher
+			http.CloseNotifier
+		}{d, &pusherDelegator{d}, &hijackerDelegator{d}, &flusherDelegator{d}, &closeNotifierDelegator{d}}
+	}
+	pickDelegator[pusher+readerFrom] = func(d *responseWriterDelegator) delegator { // 24
+		return struct {
+			*responseWriterDelegator
+			http.Pusher
+			io.ReaderFrom
+		}{d, &pusherDelegator{d}, &readerFromDelegator{d}}
+	}
+	pickDelegator[pusher+readerFrom+closeNotifier] = func(d *responseWriterDelegator) delegator { // 25
+		return struct {
+			*responseWriterDelegator
+			http.Pusher
+			io.ReaderFrom
+			http.CloseNotifier
+		}{d, &pusherDelegator{d}, &readerFromDelegator{d}, &closeNotifierDelegator{d}}
+	}
+	pickDelegator[pusher+readerFrom+flusher] = func(d *responseWriterDelegator) delegator { // 26
+		return struct {
+			*responseWriterDelegator
+			http.Pusher
+			io.ReaderFrom
+			http.Flusher
+		}{d, &pusherDelegator{d}, &readerFromDelegator{d}, &flusherDelegator{d}}
+	}
+	pickDelegator[pusher+readerFrom+flusher+closeNotifier] = func(d *responseWriterDelegator) delegator { // 27
+		return struct {
+			*responseWriterDelegator
+			http.Pusher
+			io.ReaderFrom
+			http.Flusher
+			http.CloseNotifier
+		}{d, &pusherDelegator{d}, &readerFromDelegator{d}, &flusherDelegator{d}, &closeNotifierDelegator{d}}
+	}
+	pickDelegator[pusher+readerFrom+hijacker] = func(d *responseWriterDelegator) delegator { // 28
+		return struct {
+			*responseWriterDelegator
+			http.Pusher
+			io.ReaderFrom
+			http.Hijacker
+		}{d, &pusherDelegator{d}, &readerFromDelegator{d}, &hijackerDelegator{d}}
+	}
+	pickDelegator[pusher+readerFrom+hijacker+closeNotifier] = func(d *responseWriterDelegator) delegator { // 29
+		return struct {
+			*responseWriterDelegator
+			http.Pusher
+			io.ReaderFrom
+			http.Hijacker
+			http.CloseNotifier
+		}{d, &pusherDelegator{d}, &readerFromDelegator{d}, &hijackerDelegator{d}, &closeNotifierDelegator{d}}
+	}
+	pickDelegator[pusher+readerFrom+hijacker+flusher] = func(d *responseWriterDelegator) delegator { // 30
+		return struct {
+			*responseWriterDelegator
+			http.Pusher
+			io.ReaderFrom
+			http.Hijacker
+			http.Flusher
+		}{d, &pusherDelegator{d}, &readerFromDelegator{d}, &hijackerDelegator{d}, &flusherDelegator{d}}
+	}
+	pickDelegator[pusher+readerFrom+hijacker+flusher+closeNotifier] = func(d *responseWriterDelegator) delegator { // 31
+		return struct {
+			*responseWriterDelegator
+			http.Pusher
+			io.ReaderFrom
+			http.Hijacker
+			http.Flusher
+			http.CloseNotifier
+		}{d, &pusherDelegator{d}, &readerFromDelegator{d}, &hijackerDelegator{d}, &flusherDelegator{d}, &closeNotifierDelegator{d}}
+	}
+}
+
 func newDelegator(w http.ResponseWriter, observeWriteHeaderFunc func(int)) delegator {
 	d := &responseWriterDelegator{
 		ResponseWriter:     w,
 		observeWriteHeader: observeWriteHeaderFunc,
 	}
 
-	_, cn := w.(http.CloseNotifier)
-	_, fl := w.(http.Flusher)
-	_, hj := w.(http.Hijacker)
-	_, ps := w.(http.Pusher)
-	_, rf := w.(io.ReaderFrom)
-
-	// Check for the four most common combination of interfaces a
-	// http.ResponseWriter might implement.
-	switch {
-	case cn && fl && hj && rf && ps:
-		// All interfaces.
-		return &fancyPushDelegator{
-			fancyDelegator: &fancyDelegator{d},
-			p:              &pushDelegator{d},
-		}
-	case cn && fl && hj && rf:
-		// All interfaces, except http.Pusher.
-		return &fancyDelegator{d}
-	case ps:
-		// Just http.Pusher.
-		return &pushDelegator{d}
+	id := 0
+	if _, ok := w.(http.CloseNotifier); ok {
+		id += closeNotifier
+	}
+	if _, ok := w.(http.Flusher); ok {
+		id += flusher
+	}
+	if _, ok := w.(http.Hijacker); ok {
+		id += hijacker
+	}
+	if _, ok := w.(io.ReaderFrom); ok {
+		id += readerFrom
+	}
+	if _, ok := w.(http.Pusher); ok {
+		id += pusher
 	}
 
-	return d
-}
-
-type fancyPushDelegator struct {
-	p *pushDelegator
-
-	*fancyDelegator
-}
-
-func (f *fancyPushDelegator) Push(target string, opts *http.PushOptions) error {
-	return f.p.Push(target, opts)
-}
-
-type pushDelegator struct {
-	*responseWriterDelegator
-}
-
-func (f *pushDelegator) Push(target string, opts *http.PushOptions) error {
-	return f.ResponseWriter.(http.Pusher).Push(target, opts)
+	return pickDelegator[id](d)
 }
diff --git a/prometheus/promhttp/delegator_1_7.go b/prometheus/promhttp/delegator_pre_1_8.go
similarity index 75%
rename from prometheus/promhttp/delegator_1_7.go
rename to prometheus/promhttp/delegator_pre_1_8.go
index 24b3150..8bb9b8b 100644
--- a/prometheus/promhttp/delegator_1_7.go
+++ b/prometheus/promhttp/delegator_pre_1_8.go
@@ -26,13 +26,19 @@ func newDelegator(w http.ResponseWriter, observeWriteHeaderFunc func(int)) deleg
 		observeWriteHeader: observeWriteHeaderFunc,
 	}
 
-	_, cn := w.(http.CloseNotifier)
-	_, fl := w.(http.Flusher)
-	_, hj := w.(http.Hijacker)
-	_, rf := w.(io.ReaderFrom)
-	if cn && fl && hj && rf {
-		return &fancyDelegator{d}
+	id := 0
+	if _, ok := w.(http.CloseNotifier); ok {
+		id += closeNotifier
+	}
+	if _, ok := w.(http.Flusher); ok {
+		id += flusher
+	}
+	if _, ok := w.(http.Hijacker); ok {
+		id += hijacker
+	}
+	if _, ok := w.(io.ReaderFrom); ok {
+		id += readerFrom
 	}
 
-	return d
+	return pickDelegator[id](d)
 }
diff --git a/prometheus/promhttp/instrument_server.go b/prometheus/promhttp/instrument_server.go
index ac419e5..a9b9649 100644
--- a/prometheus/promhttp/instrument_server.go
+++ b/prometheus/promhttp/instrument_server.go
@@ -14,9 +14,6 @@
 package promhttp
 
 import (
-	"bufio"
-	"io"
-	"net"
 	"net/http"
 	"strconv"
 	"strings"
@@ -435,71 +432,3 @@ func sanitizeCode(s int) string {
 		return strconv.Itoa(s)
 	}
 }
-
-type delegator interface {
-	Status() int
-	Written() int64
-
-	http.ResponseWriter
-}
-
-type responseWriterDelegator struct {
-	http.ResponseWriter
-
-	handler, method    string
-	status             int
-	written            int64
-	wroteHeader        bool
-	observeWriteHeader func(int)
-}
-
-func (r *responseWriterDelegator) Status() int {
-	return r.status
-}
-
-func (r *responseWriterDelegator) Written() int64 {
-	return r.written
-}
-
-func (r *responseWriterDelegator) WriteHeader(code int) {
-	r.status = code
-	r.wroteHeader = true
-	r.ResponseWriter.WriteHeader(code)
-	if r.observeWriteHeader != nil {
-		r.observeWriteHeader(code)
-	}
-}
-
-func (r *responseWriterDelegator) Write(b []byte) (int, error) {
-	if !r.wroteHeader {
-		r.WriteHeader(http.StatusOK)
-	}
-	n, err := r.ResponseWriter.Write(b)
-	r.written += int64(n)
-	return n, err
-}
-
-type fancyDelegator struct {
-	*responseWriterDelegator
-}
-
-func (r *fancyDelegator) CloseNotify() <-chan bool {
-	return r.ResponseWriter.(http.CloseNotifier).CloseNotify()
-}
-
-func (r *fancyDelegator) Hijack() (net.Conn, *bufio.ReadWriter, error) {
-	return r.ResponseWriter.(http.Hijacker).Hijack()
-}
-
-func (r *fancyDelegator) Flush() {
-	r.ResponseWriter.(http.Flusher).Flush()
-}
-
-func (r *fancyDelegator) ReadFrom(re io.Reader) (int64, error) {
-	if !r.wroteHeader {
-		r.WriteHeader(http.StatusOK)
-	}
-	n, err := r.ResponseWriter.(io.ReaderFrom).ReadFrom(re)
-	r.written += n
-	return n, err
-}
diff --git a/prometheus/promhttp/instrument_server_test.go b/prometheus/promhttp/instrument_server_test.go
index 7d372c4..50f7524 100644
--- a/prometheus/promhttp/instrument_server_test.go
+++ b/prometheus/promhttp/instrument_server_test.go
@@ -14,6 +14,7 @@
 package promhttp
 
 import (
+	"io"
 	"log"
 	"net/http"
 	"net/http/httptest"
@@ -105,6 +106,45 @@ func TestInstrumentTimeToFirstWrite(t *testing.T) {
 	}
 }
 
+// testResponseWriter is an http.ResponseWriter that also implements
+// http.CloseNotifier, http.Flusher, and io.ReaderFrom.
+type testResponseWriter struct {
+	closeNotifyCalled, flushCalled, readFromCalled bool
+}
+
+func (t *testResponseWriter) Header() http.Header       { return nil }
+func (t *testResponseWriter) Write([]byte) (int, error) { return 0, nil }
+func (t *testResponseWriter) WriteHeader(int)           {}
+func (t *testResponseWriter) CloseNotify() <-chan bool {
+	t.closeNotifyCalled = true
+	return nil
+}
+func (t *testResponseWriter) Flush() { t.flushCalled = true }
+func (t *testResponseWriter) ReadFrom(io.Reader) (int64, error) {
+	t.readFromCalled = true
+	return 0, nil
+}
+
+func TestInterfaceUpgrade(t *testing.T) {
+	w := &testResponseWriter{}
+	d := newDelegator(w, nil)
+	d.(http.CloseNotifier).CloseNotify()
+	if !w.closeNotifyCalled {
+		t.Error("CloseNotify not called")
+	}
+	d.(http.Flusher).Flush()
+	if !w.flushCalled {
+		t.Error("Flush not called")
+	}
+	d.(io.ReaderFrom).ReadFrom(nil)
+	if !w.readFromCalled {
+		t.Error("ReadFrom not called")
+	}
+	if _, ok := d.(http.Hijacker); ok {
+		t.Error("delegator unexpectedly implements http.Hijacker")
+	}
+}
+
 func ExampleInstrumentHandlerDuration() {
 	inFlightGauge := prometheus.NewGauge(prometheus.GaugeOpts{
 		Name: "in_flight_requests",