lib.enum: allow empty enums.

amaranth/lib/enum.py

@@ -112,16 +112,11 @@ class EnumMeta(ShapeCastable, py_enum.EnumMeta):
         if hasattr(cls, "_amaranth_shape_"):
             # Shape was provided explicitly; return it.
             return cls._amaranth_shape_
-        elif cls.__members__:
-            # Shape was not provided explicitly, but enumeration has members; treat it
-            # the same way `Shape.cast` treats standard library enumerations, so that
-            # `amaranth.lib.enum.Enum` can be a drop-in replacement for `enum.Enum`.
-            return Shape._cast_plain_enum(cls)
         else:
-            # Shape was not provided explicitly, and enumeration has no members.
+            # Shape was not provided explicitly; treat it the same way `Shape.cast` treats
-            # This is a base or mixin class that cannot be instantiated directly.
+            # standard library enumerations, so that `amaranth.lib.enum.Enum` can be a drop-in
-            raise TypeError("Enumeration '{}.{}' does not have a defined shape"
+            # replacement for `enum.Enum`.
-                            .format(cls.__module__, cls.__qualname__))
+            return Shape._cast_plain_enum(cls)
 
     def __call__(cls, value):
         # :class:`py_enum.Enum` uses ``__call__()`` for type casting: ``E(x)`` returns

tests/test_lib_enum.py

@@ -1,3 +1,5 @@
+import enum as py_enum
+
 from amaranth import *
 from amaranth.lib.enum import Enum
 
@@ -21,9 +23,10 @@ class EnumTestCase(FHDLTestCase):
     def test_shape_no_members(self):
         class EnumA(Enum):
             pass
-        with self.assertRaisesRegex(TypeError,
+        class PyEnumA(py_enum.Enum):
-                r"^Enumeration '.+?\.EnumA' does not have a defined shape$"):
+            pass
-            Shape.cast(EnumA)
+        self.assertEqual(Shape.cast(EnumA), unsigned(0))
+        self.assertEqual(Shape.cast(PyEnumA), unsigned(0))
 
     def test_shape_explicit(self):
         class EnumA(Enum, shape=signed(2)):