lib.wiring: ensure flipped(flipped(intf)) is intf.

Author: whitequark

amaranth/lib/wiring.py

@@ -552,7 +552,10 @@ def __repr__(self):
 
 
 def flipped(interface):
-    return FlippedInterface(interface)
+    if type(interface) is FlippedInterface:
+        return interface._FlippedInterface__unflipped
+    else:
+        return FlippedInterface(interface)
 
 
 @final

tests/test_lib_wiring.py

@@ -530,6 +530,7 @@ def test_basic(self):
         self.assertEqual(tintf, flipped(intf))
         self.assertRegex(repr(tintf),
             r"^flipped\(<.+?\.Interface object at .+>\)$")
+        self.assertIs(flipped(tintf), intf)
 
     def test_getattr_setattr(self):
         class I(Interface):