Visit index expressions only once

Author: brianschubert

mypy/checkexpr.py

@@ -5612,14 +5612,14 @@ def visit_slice_expr(self, e: SliceExpr) -> Type:
         except KeyError:
             supports_index = self.chk.named_type("builtins.int")  # thanks, fixture life
         expected = make_optional_type(supports_index)
+        type_args = []
         for index in [e.begin_index, e.end_index, e.stride]:
             if index:
                 t = self.accept(index)
                 self.chk.check_subtype(t, expected, index, message_registry.INVALID_SLICE_INDEX)
-        type_args = [
-            NoneType() if arg is None else self.accept(arg)
-            for arg in [e.begin_index, e.end_index, e.stride]
-        ]
+                type_args.append(t)
+            else:
+                type_args.append(NoneType())
         return self.chk.named_generic_type("builtins.slice", type_args)
 
     def visit_list_comprehension(self, e: ListComprehension) -> Type: