hdl.ast: fix Array not being indexable by ValueCastable

amaranth/hdl/ast.py

@@ -1382,6 +1382,8 @@ def __init__(self, iterable=()):
         self._mutable  = True
 
     def __getitem__(self, index):
+        if isinstance(index, ValueCastable):
+            index = Value.cast(index)
         if isinstance(index, Value):
             if self._mutable:
                 self._proxy_at = tracer.get_src_loc()

tests/test_hdl_ast.py

@@ -1005,6 +1005,18 @@ def test_becomes_immutable(self):
                 r"^Array can no longer be mutated after it was indexed with a value at "):
             a.insert(1, 2)
 
+    def test_index_value_castable(self):
+        class MyValue(ValueCastable):
+            @ValueCastable.lowermethod
+            def as_value(self):
+                return Signal()
+
+            def shape():
+                return unsigned(1)
+
+        a = Array([1,2,3])
+        a[MyValue()]
+
     def test_repr(self):
         a = Array([1,2,3])
         self.assertEqual(repr(a), "(array mutable [1, 2, 3])")