hdl.ast: change warning on out-of-range reset to an error, improve it.

Fixes #1019.

amaranth/hdl/ast.py

@@ -1170,13 +1170,14 @@ class Signal(Value, DUID, metaclass=_SignalMeta):
         self.reset = reset.value
         self.reset_less = bool(reset_less)
 
-        if isinstance(orig_shape, range) and self.reset == orig_shape.stop:
-            warnings.warn(
-                message="Reset value {!r} equals the non-inclusive end of the signal "
-                        "shape {!r}; this is likely an off-by-one error"
-                        .format(self.reset, orig_shape),
-                category=SyntaxWarning,
-                stacklevel=2)
+        if isinstance(orig_shape, range) and orig_reset is not None and orig_reset not in orig_shape:
+            if orig_reset == orig_shape.stop:
+                raise SyntaxError(
+                    f"Reset value {orig_reset!r} equals the non-inclusive end of the signal "
+                    f"shape {orig_shape!r}; this is likely an off-by-one error")
+            else:
+                raise SyntaxError(
+                    f"Reset value {orig_reset!r} is not within the signal shape {orig_shape!r}")
 
         self.attrs = OrderedDict(() if attrs is None else attrs)
 

docs/changes.rst

@@ -50,6 +50,8 @@ Language changes
 * Added: :func:`amaranth.utils.ceil_log2`, :func:`amaranth.utils.exact_log2`. (`RFC 17`_)
 * Changed: ``m.Case()`` with no patterns is never active instead of always active. (`RFC 39`_)
 * Changed: ``Value.matches()`` with no patterns is ``Const(0)`` instead of ``Const(1)``. (`RFC 39`_)
+* Changed: ``Signal(range(stop), reset=stop)`` warning has been changed into a hard error and made to trigger on any out-of range value.
+* Changed: ``Signal(range(0))`` is now valid without a warning.
 * Deprecated: :func:`amaranth.utils.log2_int`. (`RFC 17`_)
 * Removed: (deprecated in 0.4) :meth:`Const.normalize`. (`RFC 5`_)
 * Removed: (deprecated in 0.4) :class:`ast.Sample`, :class:`ast.Past`, :class:`ast.Stable`, :class:`ast.Rose`, :class:`ast.Fell`.

tests/test_hdl_ast.py

@@ -1106,10 +1106,8 @@ class SignalTestCase(FHDLTestCase):
         self.assertEqual(s8.shape(), signed(5))
         s9 = Signal(range(-20, 16))
         self.assertEqual(s9.shape(), signed(6))
-        with warnings.catch_warnings():
-            warnings.filterwarnings(action="ignore", category=SyntaxWarning)
-            s10 = Signal(range(0))
-            self.assertEqual(s10.shape(), unsigned(0))
+        s10 = Signal(range(0))
+        self.assertEqual(s10.shape(), unsigned(0))
         s11 = Signal(range(1))
         self.assertEqual(s11.shape(), unsigned(1))
 
@@ -1184,10 +1182,22 @@ class SignalTestCase(FHDLTestCase):
             Signal(signed(1), reset=-2)
 
     def test_reset_wrong_fencepost(self):
-        with self.assertWarnsRegex(SyntaxWarning,
+        with self.assertRaisesRegex(SyntaxError,
                 r"^Reset value 10 equals the non-inclusive end of the signal shape "
                 r"range\(0, 10\); this is likely an off-by-one error$"):
             Signal(range(0, 10), reset=10)
+        with self.assertRaisesRegex(SyntaxError,
+                r"^Reset value 0 equals the non-inclusive end of the signal shape "
+                r"range\(0, 0\); this is likely an off-by-one error$"):
+            Signal(range(0), reset=0)
+
+    def test_reset_wrong_range(self):
+        with self.assertRaisesRegex(SyntaxError,
+                r"^Reset value 11 is not within the signal shape range\(0, 10\)$"):
+            Signal(range(0, 10), reset=11)
+        with self.assertRaisesRegex(SyntaxError,
+                r"^Reset value 0 is not within the signal shape range\(1, 10\)$"):
+            Signal(range(1, 10), reset=0)
 
     def test_attrs(self):
         s1 = Signal()