Merge pull request #4411 from jedwards4b/exeroot_not_caseroot

do not allow EXEROOT or OBJDIR or LIBROOT to be CASEROOT

CIME/XML/env_build.py

@@ -18,4 +18,19 @@ def __init__(
         initialize an object interface to file env_build.xml in the case directory
         """
         schema = os.path.join(utils.get_schema_path(), "env_entry_id.xsd")
+        self._caseroot = case_root
         EnvBase.__init__(self, case_root, infile, schema=schema, read_only=read_only)
+
+    def set_value(self, vid, value, subgroup=None, ignore_type=False):
+        """
+        Set the value of an entry-id field to value
+        Returns the value or None if not found
+        subgroup is ignored in the general routine and applied in specific methods
+        """
+        # Do not allow any of these to be the same as CASEROOT
+        if vid in ("EXEROOT", "OBJDIR", "LIBROOT"):
+            utils.expect(value != self._caseroot, f"Cannot set {vid} to CASEROOT")
+
+        return super(EnvBuild, self).set_value(
+            vid, value, subgroup=subgroup, ignore_type=ignore_type
+        )