The hack made `0` legal against `os.Error` by blanking the constant's
`ExactValue` via `update_untyped_expr_value`, which is a silent no-op when
the expression has not been registered as an untyped expression yet. Since
#7087, `check_expr_base` calls `convert_to_typed` before `add_untyped` runs,
so the blanking is lost and the backend receives a constant typed `os.Error`
still holding integer `0` with no variant type, tripping
`GB_ASSERT_MSG(value_type != nullptr)` in `lb_const_value`.
Also drop the redundant `operand->mode = Addressing_Invalid;` before the
union `convert_untyped_error` call, which revives the `Did you want 'nil'?`
hint, and guard that hint on `type_has_nil` and an Integer/Float zero.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
