hdl.ast: Value.matches() with no arguments should return C(1).

The behavior of the following must be always the same:
- `with m.Switch(v): with m.Case(*pats):`
- `with m.If(v.matches(*pats)):`

Author: whitequark

amaranth/hdl/ast.py

@@ -442,7 +442,7 @@ def matches(self, *patterns):
                     continue
                 matches.append(self == pattern)
         if not matches:
-            return Const(0)
+            return Const(1)
         elif len(matches) == 1:
             return matches[0]
         else:

docs/changes.rst

@@ -34,6 +34,7 @@ Language changes
 * Added: :meth:`Value.matches` and ``with m.Case():`` accept any constant-castable objects. (`RFC 4`_)
 * Changed: :meth:`Value.cast` casts :class:`ValueCastable` objects recursively.
 * Changed: :meth:`Value.cast` treats instances of classes derived from both :class:`enum.Enum` and :class:`int` (including :class:`enum.IntEnum`) as enumerations rather than integers.
+* Changed: ``Value.matches()`` with an empty list of patterns returns ``Const(1)`` rather than ``Const(0)``, to match ``with m.Case():``.
 * Changed: :class:`Cat` accepts instances of classes derived from both :class:`enum.Enum` and :class:`int` (including :class:`enum.IntEnum`) without warning.
 * Deprecated: :meth:`Const.normalize`. (`RFC 5`_)
 * Removed: (deprecated in 0.1) casting of :class:`Shape` to and from a ``(width, signed)`` tuple.

tests/test_hdl_ast.py

@@ -599,7 +599,7 @@ def test_xor_value(self):
 
     def test_matches(self):
         s = Signal(4)
-        self.assertRepr(s.matches(), "(const 1'd0)")
+        self.assertRepr(s.matches(), "(const 1'd1)")
         self.assertRepr(s.matches(1), """
         (== (sig s) (const 1'd1))
         """)