fhdl.ir: a subfragment's input that we don't drive is also our input.

nmigen/fhdl/ir.py

@@ -152,8 +152,10 @@ class Fragment:
             # to provide. If the subfragment is not driving it, it will silently ignore it.
             sub_ins, sub_outs = subfrag._propagate_ports(ports=self_used | ports)
             # Refine the input port approximation: if a subfragment is driving a signal,
-            # it is definitely not our input.
+            # it is definitely not our input. But, if a subfragment requires a signal as an input,
+            # and we aren't driving it, it has to be our input as well.
             ins  -= sub_outs
+            ins  |= sub_ins - self_driven
             # Refine the output port approximation: if a subfragment is driving a signal,
             # and we're asked to provide it, we can provide it now.
             outs |= ports & sub_outs

nmigen/test/test_fhdl_ir.py

@@ -71,6 +71,19 @@ class FragmentPortsTestCase(FHDLTestCase):
         self.assertEqual(f1.ports, ValueSet())
         self.assertEqual(f2.ports, ValueSet((self.s1,)))
 
+    def test_input_only_in_subfragment(self):
+        f1 = Fragment()
+        f2 = Fragment()
+        f2.add_statements(
+            self.c1.eq(self.s1)
+        )
+        f1.add_subfragment(f2)
+        ins, outs = f1._propagate_ports(ports=())
+        self.assertEqual(ins, ValueSet((self.s1,)))
+        self.assertEqual(outs, ValueSet())
+        self.assertEqual(f1.ports, ValueSet((self.s1,)))
+        self.assertEqual(f2.ports, ValueSet((self.s1,)))
+
     def test_output_from_subfragment(self):
         f1 = Fragment()
         f1.add_statements(