src/Pure/Admin/build_release.scala

     Isabelle_System.gnutar(args, cwd = dir.file).check
 
   private def tar_options: String =
     if (Platform.is_macos) "--owner=root --group=staff" else "--owner=root --group=root"
 
-  private val default_platform_families =
+  private val default_platform_families: List[Platform.Family.Value] =
     List(Platform.Family.linux, Platform.Family.windows, Platform.Family.macos)
 
   def build_release(base_dir: Path,
     options: Options,
     components_base: Option[Path] = None,

       val more_args = getopts(args)
       val base_dir = more_args match { case List(base_dir) => base_dir case _ => getopts.usage() }
 
       val progress = new Console_Progress()
 
+      if (platform_families.contains(Platform.Family.windows) && !Isabelle_System.bash("7z i").ok)
+        error("Building for windows requires 7z")
+
       build_release(Path.explode(base_dir), options, components_base = components_base,
         progress = progress, rev = rev, afp_rev = afp_rev, official_release = official_release,
         proper_release_name = proper_release_name, website = website,
         platform_families =
           if (platform_families.isEmpty) default_platform_families