hdl.ast: suggest bit_select or word_select when indexing with Value.

Fixes #1044.

amaranth/hdl/ast.py

@@ -370,10 +370,14 @@ def __getitem__(self, key):
                 key += n
             return Slice(self, key, key + 1, src_loc_at=1)
         elif isinstance(key, slice):
+            if isinstance(key.start, Value) or isinstance(key.stop, Value):
+                raise TypeError(f"Cannot slice value with a value; use Value.bit_select() or Value.word_select() instead")
             start, stop, step = key.indices(n)
             if step != 1:
                 return Cat(self[i] for i in range(start, stop, step))
             return Slice(self, start, stop, src_loc_at=1)
+        elif isinstance(key, Value):
+            raise TypeError(f"Cannot index value with a value; use Value.bit_select() instead")
         else:
             raise TypeError(f"Cannot index value with {key!r}")
 

tests/test_hdl_ast.py

@@ -303,6 +303,15 @@ def test_getitem_wrong(self):
         with self.assertRaisesRegex(TypeError,
                 r"^Cannot index value with 'str'$"):
             Const(31)["str"]
+        with self.assertRaisesRegex(TypeError,
+                r"^Cannot index value with a value; use Value.bit_select\(\) instead$"):
+            Const(31)[Signal(3)]
+        s = Signal(3)
+        with self.assertRaisesRegex(TypeError,
+                r"^Cannot slice value with a value; use Value.bit_select\(\) or Value.word_select\(\) instead$"):
+            Const(31)[s:s+3]
+
+
 
     def test_shift_left(self):
         self.assertRepr(Const(256, unsigned(9)).shift_left(0),