parsec: catch section/option conflict in validation

* Closes #4955
* If an item in the spec is a section but the user specified an option,
  raise an IllegalItemError if it is.

CHANGES.md

@@ -14,6 +14,10 @@ ones in. -->
 
 ### Fixes
 
+[5450](https://github.com/cylc/cylc-flow/pull/5450) - Validation will now
+fail earlier for cases where a configuration section is set as an option
+(e.g. `scheudler = 42`).
+
 [5398](https://github.com/cylc/cylc-flow/pull/5398) - Fix platform from
 group selection order bug.
 

cylc/flow/parsec/validate.py

@@ -204,10 +204,14 @@ def validate(self, cfg_root, spec_root):
                 else:
                     speckey = key
                 specval = spec[speckey]
-                if isinstance(value, dict) and not specval.is_leaf():
+
+                cfg_is_section = isinstance(value, dict)
+                spec_is_section = not specval.is_leaf()
+
+                if cfg_is_section and spec_is_section:
                     # Item is dict, push to queue
                     queue.append([value, specval, keys + [key]])
-                elif value is not None and specval.is_leaf():
+                if value is not None and not spec_is_section:
                     # Item is value, coerce according to value type
                     cfg[key] = self.coercers[specval.vdr](value, keys + [key])
                     if specval.options:
@@ -218,6 +222,12 @@ def validate(self, cfg_root, spec_root):
                                 cfg[key] not in voptions):
                             raise IllegalValueError(
                                 'option', keys + [key], cfg[key])
+                elif spec_is_section and not cfg_is_section:
+                    raise IllegalItemError(
+                        keys,
+                        key,
+                        msg=f'{key} should be a section not an option',
+                    )
 
     __call__ = validate
 

tests/functional/validate/76-section-as-setting.t

@@ -0,0 +1,38 @@
+#!/usr/bin/env bash
+# THIS FILE IS PART OF THE CYLC WORKFLOW ENGINE.
+# Copyright (C) NIWA & British Crown (Met Office) & Contributors.
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#-------------------------------------------------------------------------------
+# Test CYLC_TEMPLATE_VARS exported.
+
+. "$(dirname "$0")/test_header"
+
+set_test_number 2
+
+cat > 'flow.cylc' <<__HEREDOC__
+meta = 22
+
+[scheduling]
+  [[graph]]
+    R1 = foo
+
+[runtime]
+  [[foo]]
+__HEREDOC__
+
+run_fail "${TEST_NAME_BASE}-validate" cylc validate .
+grep_ok  \
+    "IllegalItemError:.*meta" \
+    "${TEST_NAME_BASE}-validate.stderr"