diff --git a/amaranth/hdl/ast.py b/amaranth/hdl/ast.py
index eb29620..bf3f526 100644
--- a/amaranth/hdl/ast.py
+++ b/amaranth/hdl/ast.py
@@ -41,12 +41,10 @@ class ShapeCastable:
     a richer description of the shape than what is supported by the core Amaranth language, yet
     still be transparently used with it.
     """
-    def __new__(cls, *args, **kwargs):
-        self = super().__new__(cls)
-        if not hasattr(self, "as_shape"):
+    def __init_subclass__(cls, **kwargs):
+        if not hasattr(cls, "as_shape"):
             raise TypeError(f"Class '{cls.__name__}' deriving from `ShapeCastable` must override "
                             f"the `as_shape` method")
-        return self
 
 
 class Shape:
@@ -1319,15 +1317,13 @@ class ValueCastable:
     from :class:`ValueCastable` is mutable, it is up to the user to ensure that it is not mutated
     in a way that changes its representation after the first call to :meth:`as_value`.
     """
-    def __new__(cls, *args, **kwargs):
-        self = super().__new__(cls)
-        if not hasattr(self, "as_value"):
+    def __init_subclass__(cls, **kwargs):
+        if not hasattr(cls, "as_value"):
             raise TypeError(f"Class '{cls.__name__}' deriving from `ValueCastable` must override "
                             "the `as_value` method")
-        if not hasattr(self.as_value, "_ValueCastable__memoized"):
+        if not hasattr(cls.as_value, "_ValueCastable__memoized"):
             raise TypeError(f"Class '{cls.__name__}' deriving from `ValueCastable` must decorate "
                             "the `as_value` method with the `ValueCastable.lowermethod` decorator")
-        return self
 
     @staticmethod
     def lowermethod(func):
diff --git a/tests/test_hdl_ast.py b/tests/test_hdl_ast.py
index 3d18091..7d95e20 100644
--- a/tests/test_hdl_ast.py
+++ b/tests/test_hdl_ast.py
@@ -145,17 +145,14 @@ class MockShapeCastable(ShapeCastable):
         return self.dest
 
 
-class MockShapeCastableNoOverride(ShapeCastable):
-    def __init__(self):
-        pass
-
-
 class ShapeCastableTestCase(FHDLTestCase):
     def test_no_override(self):
         with self.assertRaisesRegex(TypeError,
                 r"^Class 'MockShapeCastableNoOverride' deriving from `ShapeCastable` must "
                 r"override the `as_shape` method$"):
-            sc = MockShapeCastableNoOverride()
+            class MockShapeCastableNoOverride(ShapeCastable):
+                def __init__(self):
+                    pass
 
     def test_cast(self):
         sc = MockShapeCastable(unsigned(2))
@@ -1135,19 +1132,6 @@ class MockValueCastableChanges(ValueCastable):
         return Signal(self.width)
 
 
-class MockValueCastableNotDecorated(ValueCastable):
-    def __init__(self):
-        pass
-
-    def as_value(self):
-        return Signal()
-
-
-class MockValueCastableNoOverride(ValueCastable):
-    def __init__(self):
-        pass
-
-
 class MockValueCastableCustomGetattr(ValueCastable):
     def __init__(self):
         pass
@@ -1165,13 +1149,20 @@ class ValueCastableTestCase(FHDLTestCase):
         with self.assertRaisesRegex(TypeError,
                 r"^Class 'MockValueCastableNotDecorated' deriving from `ValueCastable` must "
                 r"decorate the `as_value` method with the `ValueCastable.lowermethod` decorator$"):
-            vc = MockValueCastableNotDecorated()
+            class MockValueCastableNotDecorated(ValueCastable):
+                def __init__(self):
+                    pass
+
+                def as_value(self):
+                    return Signal()
 
     def test_no_override(self):
         with self.assertRaisesRegex(TypeError,
                 r"^Class 'MockValueCastableNoOverride' deriving from `ValueCastable` must "
                 r"override the `as_value` method$"):
-            vc = MockValueCastableNoOverride()
+            class MockValueCastableNoOverride(ValueCastable):
+                def __init__(self):
+                    pass
 
     def test_memoized(self):
         vc = MockValueCastableChanges(1)