// ErrContentLength is returned by ResponseWriter.Write calls
// when a Handler set a Content-Length response header with a
// declared size and then attempted to write more bytes than
// declared.
ErrContentLength = errors.New("http: wrote more than the declared Content-Length")

Writing too many bytes to an HTTP/2 body doesn't return ErrContentLength , however, for two reasons:

1. The HTTP/2 ResponseWriter returns a different error when detecting a too-long body.
2. The accounting for how many bytes have been written can fail to detect a too-long body until after the ResponseWriter.Write call has returned.
   The existing test TestIdentityResponse fails when run in HTTP/2 mode.