diff --git a/actix-http/src/body/boxed.rs b/actix-http/src/body/boxed.rs
index d2469e986..d4737aab8 100644
--- a/actix-http/src/body/boxed.rs
+++ b/actix-http/src/body/boxed.rs
@@ -57,27 +57,7 @@ impl MessageBody for BoxBody {
     }
 
     fn take_complete_body(&mut self) -> Bytes {
-        debug_assert!(
-            self.is_complete_body(),
-            "boxed type does not allow taking complete body; caller should make sure to \
-            call `is_complete_body` first",
-        );
-
-        // we do not have DerefMut access to call take_complete_body directly but since
-        // is_complete_body is true we should expect the entire bytes chunk in one poll_next
-
-        let waker = futures_util::task::noop_waker();
-        let mut cx = Context::from_waker(&waker);
-
-        match self.as_pin_mut().poll_next(&mut cx) {
-            Poll::Ready(Some(Ok(data))) => data,
-            _ => {
-                panic!(
-                    "boxed type indicated it allows taking complete body but failed to \
-                    return Bytes when polled",
-                );
-            }
-        }
+        self.0.take_complete_body()
     }
 }
 
diff --git a/actix-http/src/body/message_body.rs b/actix-http/src/body/message_body.rs
index 3e6c8d5cb..20263b3fb 100644
--- a/actix-http/src/body/message_body.rs
+++ b/actix-http/src/body/message_body.rs
@@ -134,7 +134,7 @@ mod foreign_impls {
 
     impl<B> MessageBody for Box<B>
     where
-        B: MessageBody + Unpin,
+        B: MessageBody + Unpin + ?Sized,
     {
         type Error = B::Error;
 
@@ -164,7 +164,7 @@ mod foreign_impls {
 
     impl<B> MessageBody for Pin<Box<B>>
     where
-        B: MessageBody,
+        B: MessageBody + ?Sized,
     {
         type Error = B::Error;
 
@@ -175,10 +175,10 @@ mod foreign_impls {
 
         #[inline]
         fn poll_next(
-            mut self: Pin<&mut Self>,
+            self: Pin<&mut Self>,
             cx: &mut Context<'_>,
         ) -> Poll<Option<Result<Bytes, Self::Error>>> {
-            self.as_mut().poll_next(cx)
+            self.get_mut().as_mut().poll_next(cx)
         }
 
         #[inline]
@@ -475,6 +475,16 @@ where
             None => Poll::Ready(None),
         }
     }
+
+    #[inline]
+    fn is_complete_body(&self) -> bool {
+        self.body.is_complete_body()
+    }
+
+    #[inline]
+    fn take_complete_body(&mut self) -> Bytes {
+        self.body.take_complete_body()
+    }
 }
 
 #[cfg(test)]
@@ -630,6 +640,27 @@ mod tests {
         assert_eq!(Pin::new(&mut data).poll_next(&mut cx), Poll::Ready(None));
     }
 
+    #[test]
+    fn complete_body_combinators() {
+        use crate::body::{BoxBody, EitherBody};
+
+        let body = Bytes::from_static(b"test");
+        let body = BoxBody::new(body);
+        let body = EitherBody::<_, ()>::left(body);
+        let body = EitherBody::<(), _>::right(body);
+        let body = Box::new(body);
+        let body = Box::pin(body);
+        let mut body = body;
+
+        assert!(body.is_complete_body());
+        assert_eq!(body.take_complete_body(), b"test".as_ref());
+
+        // subsequent poll_next returns None
+        let waker = futures_util::task::noop_waker();
+        let mut cx = Context::from_waker(&waker);
+        assert!(Pin::new(&mut body).poll_next(&mut cx).map_err(drop) == Poll::Ready(None));
+    }
+
     // down-casting used to be done with a method on MessageBody trait
     // test is kept to demonstrate equivalence of Any trait
     #[actix_rt::test]