If you are using a slide film, you expose for the highlights, i.e., you take care not to blow highlights. You let the shadows fall where they may. Shadows can be blocked because slide films have less dynamic range than negative films. The final result will typically be unpleasant with blocked shadows, and disastrous with blown highlights. You would go for the lesser evil.
If you own a spot light meter you can measure the exposure range, figure out the exposure "for the highlights", check where the shadows fall, visualize the final result, and if it is awful you save the slide film (you use a negative film if you can, or you give up the picture, or you reduce the contrast, by using fill lights for instance if possible).