lib.data: allow Const as value of Layout.const(...) field.

Fixes #838.

amaranth/lib/data.py

@@ -238,7 +238,7 @@ class Layout(ShapeCastable, metaclass=ABCMeta):
                                      "it casts to, {!r}, and not {!r}"
                                      .format(field.shape, cast_field_shape,
                                              key_value.shape()))
-            else:
+            elif not isinstance(key_value, Const):
                 key_value = Const(key_value, cast_field_shape)
             int_value &= ~(((1 << cast_field_shape.width) - 1) << field.offset)
             int_value |= key_value.value << field.offset

tests/test_lib_data.py

@@ -424,6 +424,10 @@ class LayoutTestCase(FHDLTestCase):
                 r"that it casts to, unsigned\(8\), and not unsigned\(1\)$"):
             sl.const({"f": "01"})
 
+    def test_const_field_const(self):
+        sl = StructLayout({"f": unsigned(1)})
+        self.assertRepr(sl.const({"f": Const(1)}), "(const 1'd1)")
+
     def test_signal_reset(self):
         sl = StructLayout({
             "a": unsigned(1),