fix

cylc/flow/config.py

@@ -2262,6 +2262,7 @@ def load_graph(self):
             self.workflow_polling_tasks.update(
                 parser.workflow_state_polling_tasks)
             self._proc_triggers(parser, seq, task_triggers)
+            self.check_outputs(parser.terminals)
 
         self.set_required_outputs(task_output_opt)
 
@@ -2275,20 +2276,21 @@ def load_graph(self):
         for tdef in self.taskdefs.values():
             tdef.tweak_outputs()
 
-    def check_outputs(self, tasks_and_ouputs):
+    def check_outputs(self, terminals):
         """Check that task outputs have been registered with tasks.
-
-        TODO - have a bit more of a think about whether the try except is
-        safe
         """
-        for task, output in tasks_and_ouputs:
+        for terminal in terminals:
+            parts = terminal.split(':')
+            if len(parts) != 2:
+                continue
+            task, output = parts
             try:
                 registered_outputs = self.cfg['runtime'][task]['outputs']
             except KeyError:
                 registered_outputs = []
             if (
-                not TaskOutputs.is_valid_std_name(output)
-                and output not in registered_outputs
+                not TaskOutputs.is_valid_std_name(output.strip('?'))
+                and output.strip('?') not in registered_outputs
             ):
                 raise WorkflowConfigError(
                     f"Undefined custom output: {task}:{output}"

cylc/flow/graph_parser.py

@@ -470,10 +470,12 @@ def parse_graph(self, graph_string: str) -> None:
                 pairs.add((chain[i], chain[i + 1]))
 
         # Get a set of RH nodes which are not at the LH of another pair:
-        terminals = {p[1] for p in pairs}.difference({p[0] for p in pairs})
+        self.terminals = {p[1] for p in pairs}.difference(
+            {p[0] for p in pairs}
+        )
 
         for pair in sorted(pairs, key=lambda p: str(p[0])):
-            self._proc_dep_pair(pair, terminals)
+            self._proc_dep_pair(pair, self.terminals)
 
     @classmethod
     def _report_invalid_lines(cls, lines: List[str]) -> None: