diff -r d92e0197ba01 -r fc4d07587695 src/Pure/Thy/sessions.scala
--- a/src/Pure/Thy/sessions.scala	Tue Mar 22 18:52:27 2022 +0100
+++ b/src/Pure/Thy/sessions.scala	Tue Mar 22 18:56:28 2022 +0100
@@ -1013,7 +1013,7 @@
   def is_session_dir(dir: Path): Boolean =
     (dir + ROOT).is_file || (dir + ROOTS).is_file
 
-  private def check_session_dir(dir: Path): Path =
+  def check_session_dir(dir: Path): Path =
     if (is_session_dir(dir)) File.pwd() + dir.expand
     else error("Bad session root directory (missing ROOT or ROOTS): " + dir.expand.toString)