ast: fix const-castable expression handling in Signal(reset=).

The code to accept const-castable expressions was previously added in
0c4fda92fe, but it was untested and had
a few bugs.

Fixes #911.

amaranth/hdl/ast.py

@@ -1056,12 +1056,15 @@ class Signal(Value, DUID, metaclass=_SignalMeta):
                                  .format(orig_shape, Shape.cast(orig_shape),
                                          reset.shape()))
         else:
+            if reset is None:
+                reset = 0
             try:
-                reset = Const.cast(reset or 0)
+                reset = Const.cast(reset)
             except TypeError:
                 raise TypeError("Reset value must be a constant-castable expression, not {!r}"
                                 .format(orig_reset))
-        if orig_reset not in (None, 0, -1): # Avoid false positives for all-zeroes and all-ones
+        # Avoid false positives for all-zeroes and all-ones
+        if orig_reset is not None and not (isinstance(orig_reset, int) and orig_reset in (0, -1)):
             if reset.shape().signed and not self.signed:
                 warnings.warn(
                     message="Reset value {!r} is signed, but the signal shape is {!r}"

tests/test_hdl_ast.py

@@ -1053,6 +1053,10 @@ class SignalTestCase(FHDLTestCase):
                 r"not <StringEnum\.FOO: 'a'>$"):
             Signal(1, reset=StringEnum.FOO)
 
+    def test_reset_const_castable(self):
+        s1 = Signal(4, reset=Cat(Const(0, 1), Const(1, 1), Const(0, 2)))
+        self.assertEqual(s1.reset, 2)
+
     def test_reset_shape_castable_const(self):
         class CastableFromHex(ShapeCastable):
             def as_shape(self):