Fix gap in validation of xtrigger sequential argument

cylc/flow/xtrigger_mgr.py

@@ -325,11 +325,16 @@ def _handle_sequential_kwarg(
                 )
             fctx.func_kwargs.setdefault('sequential', sequential_param.default)
 
-        elif 'sequential' in fctx.func_kwargs:
-            # xtrig marked as sequential, so add 'sequential' arg to signature
-            sig = add_kwarg_to_sig(
-                sig, 'sequential', fctx.func_kwargs['sequential']
-            )
+        if 'sequential' in fctx.func_kwargs:
+            # xtrig marked as sequential in function call
+            value = fctx.func_kwargs['sequential']
+            if not isinstance(value, bool):
+                raise XtriggerConfigError(
+                    label, fctx.func_name,
+                    f"invalid argument 'sequential={value}' - must be boolean"
+                )
+            if not sequential_param:
+                sig = add_kwarg_to_sig(sig, 'sequential', value)
         return sig
 
     @staticmethod

tests/integration/test_sequential_xtriggers.py

@@ -159,10 +159,8 @@ async def test_sequential_arg_ok(
         assert len(list_cycles(schd)) == expected_num_cycles
 
 
-def test_sequential_arg_bad(
-    flow, validate
-):
-    """Test validation of 'sequential' arg for custom xtriggers"""
+def test_sequential_arg_bad(flow, validate):
+    """Test validation of 'sequential' arg for custom xtrigger function def"""
     wid = flow({
         'scheduling': {
             'xtriggers': {
@@ -194,6 +192,27 @@ def xtrig2(x, sequential='True'):
             ) in str(excinfo.value)
 
 
+def test_sequential_arg_bad2(flow, validate):
+    """Test validation of 'sequential' arg for xtrigger calls"""
+    wid = flow({
+        'scheduling': {
+            'initial cycle point': '2000',
+            'xtriggers': {
+                'clock': 'wall_clock(sequential=3)',
+            },
+            'graph': {
+                'R1': '@clock => foo',
+            },
+        },
+    })
+
+    with pytest.raises(XtriggerConfigError) as excinfo:
+        validate(wid)
+    assert (
+        "invalid argument 'sequential=3' - must be boolean"
+    ) in str(excinfo.value)
+
+
 @pytest.mark.parametrize('is_sequential', [True, False])
 async def test_any_sequential(flow, scheduler, start, is_sequential: bool):
     """Test that a task is marked as sequential if any of its xtriggers are."""