lib.data: improve reset value handling for Union.

* Reject union initialization with more than one reset value.
* Replace the reset value specified in the class definition with
  the one provided during initalization instead of merging.

amaranth/lib/data.py

@@ -431,10 +431,15 @@ class _AggregateMeta(ShapeCastable, type):
 class _Aggregate(View, metaclass=_AggregateMeta):
     def __init__(self, target=None, *, name=None, reset=None, reset_less=None,
             attrs=None, decoder=None, src_loc_at=0):
+        if self.__class__._AggregateMeta__layout_cls is UnionLayout:
+            if reset is not None and len(reset) > 1:
+                raise ValueError("Reset value for at most one field can be provided for "
+                                 "a union class (specified: {})"
+                                 .format(", ".join(reset.keys())))
         if target is None and hasattr(self.__class__, "_AggregateMeta__reset"):
             if reset is None:
                 reset = self.__class__._AggregateMeta__reset
-            else:
+            elif self.__class__._AggregateMeta__layout_cls is not UnionLayout:
                 reset = {**self.__class__._AggregateMeta__reset, **reset}
         super().__init__(self.__class__, target, name=name, reset=reset, reset_less=reset_less,
             attrs=attrs, decoder=decoder, src_loc_at=src_loc_at + 1)

tests/test_lib_data.py

@@ -669,6 +669,24 @@ class UnionTestCase(FHDLTestCase):
         self.assertEqual(s.attrs, {"debug": 1})
         self.assertEqual(s.decoder, decoder)
 
+    def test_construct_reset_two_wrong(self):
+        class U(Union):
+            a: unsigned(1)
+            b: unsigned(2)
+
+        with self.assertRaisesRegex(ValueError,
+                r"^Reset value for at most one field can be provided for a union class "
+                r"\(specified: a, b\)$"):
+            U(reset=dict(a=1, b=2))
+
+    def test_construct_reset_override(self):
+        class U(Union):
+            a: unsigned(1) = 1
+            b: unsigned(2)
+
+        self.assertEqual(U().as_value().reset, 0b01)
+        self.assertEqual(U(reset=dict(b=0b10)).as_value().reset, 0b10)
+
 
 # Examples from https://github.com/amaranth-lang/amaranth/issues/693
 class RFCExamplesTestCase(TestCase):