From abd74ead55c3a0d43a0f8c1b30f34ca3ace53f3a Mon Sep 17 00:00:00 2001
From: "William D. Jones" <thor0505@comcast.net>
Date: Wed, 15 Nov 2023 18:03:13 -0500
Subject: [PATCH] lib.wiring: flip sub-interfaces accessed via
 FlippedInterface.

---
 amaranth/lib/wiring.py   |  3 +++
 tests/test_lib_wiring.py | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/amaranth/lib/wiring.py b/amaranth/lib/wiring.py
index d808de6..e34d895 100644
--- a/amaranth/lib/wiring.py
+++ b/amaranth/lib/wiring.py
@@ -570,6 +570,9 @@ class FlippedInterface:
         value = getattr(self.__unflipped, name)
         if inspect.ismethod(value):
             return types.MethodType(value.__func__, self)
+        elif name in self.__unflipped.signature.members and \
+                self.__unflipped.signature.members[name].is_signature:
+            return flipped(value)
         else:
             return value
 
diff --git a/tests/test_lib_wiring.py b/tests/test_lib_wiring.py
index d7bc2ef..a1f5262 100644
--- a/tests/test_lib_wiring.py
+++ b/tests/test_lib_wiring.py
@@ -598,6 +598,38 @@ class FlippedInterfaceTestCase(unittest.TestCase):
         flipped_interface = CustomSignature({}).flip().create()
         self.assertTrue(hasattr(flipped_interface, "custom_method"))
 
+    def test_propagate_flipped(self):
+        class InterfaceWithFlippedSub(Component):
+            signature = Signature({
+                "a": In(Signature({
+                    "b": Out(Signature({
+                        "c": Out(1)
+                    })),
+                    "d": In(Signature({
+                        "e": Out(1)
+                    })),
+                    "f": Out(1)
+                }))
+            })
+
+            def __init__(self):
+                super().__init__()
+                self.g = Signature({"h": In(1)})
+
+        ifsub = InterfaceWithFlippedSub()
+        self.assertIsInstance(ifsub.a.b.signature, FlippedSignature)
+        self.assertIsInstance(ifsub.a.d.signature, Signature)
+        self.assertIsInstance(ifsub.signature.members["a"].signature.
+                              members["b"].signature, FlippedSignature)
+        self.assertIsInstance(ifsub.signature.members["a"].signature.
+                              members["d"].signature, Signature)
+        self.assertIsInstance(ifsub.a.f, Signal)
+        self.assertEqual(ifsub.signature.members["a"].signature.
+                         members["f"].flow, In)
+        self.assertIsInstance(flipped(ifsub).g, Signature)
+        self.assertEqual(ifsub.g.members["h"].flow, In)
+        self.assertEqual(flipped(ifsub).g.members["h"].flow, In)
+
 
 class ConnectTestCase(unittest.TestCase):
     def test_arg_handles_and_signature_attr(self):