// See RFC 7231, section 7.1.2
		if u.Scheme == "" && u.Host == "" {
			oldpath := r.URL.Path
			if oldpath == "" { // should not happen, but avoid a crash if it does
				oldpath = "/"
			}

			// no leading http://server
			if url == "" || url[0] != '/' {
				// make relative path absolute
				olddir, _ := path.Split(oldpath)
				url = olddir + url
			}

			var query string

	// If modifying, consider changing checkPathCollisions() in
	// src/cmd/go/internal/modcmd/vendor.go
	fold := str.ToFold(p.ImportPath)
	if other := foldPath[fold]; other == "" {
		foldPath[fold] = p.ImportPath
	} else if other != p.ImportPath {
		setError(ImportErrorf(p.ImportPath, "case-insensitive import collision: %q and %q", p.ImportPath, other))
		return
	}