From 7e438180e08f72fd5d960d279bc4f0051bdb5e42 Mon Sep 17 00:00:00 2001
From: Charlotte <charlotte@hrzn.ee>
Date: Sun, 2 Jul 2023 21:39:10 +1000
Subject: [PATCH] lib.enum: allow empty enums.

---
 amaranth/lib/enum.py   | 13 ++++---------
 tests/test_lib_enum.py |  9 ++++++---
 2 files changed, 10 insertions(+), 12 deletions(-)

diff --git a/amaranth/lib/enum.py b/amaranth/lib/enum.py
index 61e5f28..10c3666 100644
--- a/amaranth/lib/enum.py
+++ b/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.
-            # This is a base or mixin class that cannot be instantiated directly.
-            raise TypeError("Enumeration '{}.{}' does not have a defined shape"
-                            .format(cls.__module__, cls.__qualname__))
+            # Shape was not provided explicitly; 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)
 
     def __call__(cls, value):
         # :class:`py_enum.Enum` uses ``__call__()`` for type casting: ``E(x)`` returns
diff --git a/tests/test_lib_enum.py b/tests/test_lib_enum.py
index 2bf2d19..9d5a27e 100644
--- a/tests/test_lib_enum.py
+++ b/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,
-                r"^Enumeration '.+?\.EnumA' does not have a defined shape$"):
-            Shape.cast(EnumA)
+        class PyEnumA(py_enum.Enum):
+            pass
+        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)):