hdl.ast: fix Const.cast(Cat(...)) handling for signed numbers.

amaranth/hdl/ast.py

@@ -727,7 +727,8 @@ class Const(Value):
             width = 0
             for part in obj.parts:
                 const  = Const.cast(part)
-                value |= const.value << width
+                part_value = Const(const.value, unsigned(const.width)).value
+                value |= part_value << width
                 width += len(const)
             return Const(value, width)
         else:

tests/test_hdl_ast.py

@@ -935,6 +935,16 @@ class CatTestCase(FHDLTestCase):
                 r"specify the width explicitly using C\(2, 2\)$"):
             Cat(2)
 
+    def test_const(self):
+        a = Const.cast(Cat(Const(1, 1), Const(0, 1), Const(3, 2), Const(2, 2)))
+        self.assertEqual(a.value, 0x2d)
+        self.assertEqual(a.width, 6)
+        self.assertEqual(a.signed, False)
+        a = Const.cast(Cat(Const(-4, 8), Const(-3, 8)))
+        self.assertEqual(a.value, 0xfdfc)
+        self.assertEqual(a.width, 16)
+        self.assertEqual(a.signed, False)
+
 
 class ReplTestCase(FHDLTestCase):
     @_ignore_deprecated