prevent defines without an = sign being accepted.

CHANGES.md

@@ -6,6 +6,14 @@ creating a new release entry be sure to copy & paste the span tag with the
 updated. Only the first match gets replaced, so it's fine to leave the old
 ones in. -->
 
+## __cylc-rose-1.2.1 (<span actions:bind='release-date'>Upcoming</span>)__
+
+
+### Fixes
+
+[#225](https://github.com/cylc/cylc-rose/pull/225) - Prevent totally invalid
+CLI --defines with no = sign.
+
 ## __cylc-rose-1.2.0 (<span actions:bind='release-date'>Released 2023-01-16</span>)__
 
 ### Fixes

cylc/rose/entry_points.py

@@ -31,13 +31,15 @@
     dump_rose_log,
     get_rose_vars_from_config_node,
     identify_templating_section,
+    invalid_defines_check,
     rose_config_exists,
     rose_config_tree_loader,
     merge_rose_cylc_suite_install_conf,
     paths_to_pathlib,
     get_cli_opts_node,
     add_cylc_install_to_rose_conf_node_opts,
 )
+from cylc.flow.flags import cylc7_back_compat
 from cylc.flow.hostuserutil import get_host
 
 
@@ -103,6 +105,12 @@ def get_rose_vars(srcdir=None, opts=None):
                 }
             }
     """
+
+    # Check for definately invalid defines
+
+    if opts and hasattr(opts, 'defines') and not cylc7_back_compat:
+        invalid_defines_check(opts.defines)
+
     # Set up blank page for returns.
     config = {
         'env': {},

cylc/rose/utilities.py

@@ -25,6 +25,7 @@
 
 from cylc.flow.hostuserutil import get_host
 from cylc.flow import LOG
+from cylc.flow.exceptions import CylcError
 import cylc.flow.flags as flags
 from cylc.rose.jinja2_parser import Parser, patch_jinja2_leading_zeros
 from metomi.rose import __version__ as ROSE_VERSION
@@ -44,7 +45,11 @@
 )
 
 
-class MultipleTemplatingEnginesError(Exception):
+class MultipleTemplatingEnginesError(CylcError):
+    ...
+
+
+class InvalidDefineError(CylcError):
     ...
 
 
@@ -325,11 +330,42 @@ def merge_rose_cylc_suite_install_conf(old, new):
     return old
 
 
+def invalid_defines_check(defines: List) -> None:
+    """Check for defines which do not contain an = and therefore cannot be
+    valid
+
+    Examples:
+
+        # A single invalid define:
+        >>> import pytest
+        >>> with pytest.raises(InvalidDefineError, match=r'\\* foo'):
+        ...    invalid_defines_check(['foo'])
+
+        # Two invalid defines and one valid one:
+        >>> with pytest.raises(
+        ...     InvalidDefineError, match=r'\\* foo.*\\n.* \\* bar'
+        ... ):
+        ...     invalid_defines_check(['foo', 'bar52', 'baz=442'])
+
+        # No invalid defines
+        >>> invalid_defines_check(['foo=12'])
+    """
+    invalid_defines = []
+    for define in defines:
+        if parse_cli_defines(define) == 'Invalid':
+            invalid_defines.append(define)
+    if invalid_defines:
+        msg = 'Invalid Suite Defines (should contain an =)'
+        for define in invalid_defines:
+            msg += f'\n * {define}'
+        raise InvalidDefineError(msg)
+
+
 def parse_cli_defines(define: str) -> Union[
-    bool, Tuple[
+    bool, str, Tuple[
         List[Union[str, Any]],
         Union[str, Any],
-        Union[str, Any]
+        Union[str, Any],
     ]
 ]:
     """Parse a define string.
@@ -381,7 +417,7 @@ def parse_cli_defines(define: str) -> Union[
             # This seems like it ought to be an error,
             # But behaviour is consistent with Rose 2019
             # See:
-            return False
+            return "Invalid"
 
     if match['state'] in ['!', '!!']:
         LOG.warning(