hdl._ast: fix using 0-width Switch with integer keys.

This comes up in `AssignmentLegalizer`-produced `Switch`es for
`ArrayProxy`.

Author: wanda-phi

amaranth/hdl/ast.py

@@ -1769,8 +1769,14 @@ def __init__(self, test, cases, *, src_loc=None, src_loc_at=0, case_src_locs={})
                     key = "".join(key.split()) # remove whitespace
                 elif isinstance(key, int):
                     key = format(key & key_mask, "b").rjust(len(self.test), "0")
+                    # fixup for 0-width test
+                    if key_mask == 0:
+                        key = ""
                 elif isinstance(key, Enum):
                     key = format(key.value & key_mask, "b").rjust(len(self.test), "0")
+                    # fixup for 0-width test
+                    if key_mask == 0:
+                        key = ""
                 else:
                     raise TypeError("Object {!r} cannot be used as a switch key"
                                     .format(key))

tests/test_hdl_ast.py

@@ -1471,6 +1471,16 @@ def test_int_neg_case(self):
         s = Switch(Const(0, 8), {-10: []})
         self.assertEqual(s.cases, {("11110110",): []})
 
+    def test_int_zero_width(self):
+        s = Switch(Const(0, 0), {0: []})
+        self.assertEqual(s.cases, {("",): []})
+
+    def test_int_zero_width_enum(self):
+        class ZeroEnum(Enum):
+            A = 0
+        s = Switch(Const(0, 0), {ZeroEnum.A: []})
+        self.assertEqual(s.cases, {("",): []})
+
     def test_enum_case(self):
         s = Switch(Const(0, UnsignedEnum), {UnsignedEnum.FOO: []})
         self.assertEqual(s.cases, {("01",): []})