I'm doing this:

export const getSigningKey = (
        secretKey: string | pulumi.Output<string>,
        region: pulumi.Output<digitalocean.Region | aws.Region>
) => {...};

if (cloud === 'digitalocean') {
  s3 = new digitalocean.SpacesBucket(env, {
    region: awsRegion,
  });
} else if (cloud === 'aws') {
  s3 = new aws.s3.Bucket(env, {
    region: awsRegion,
  });
}
signingKey = getSigningKey(
  awsSecretKey,
  s3!.region
);

this doesn't work. it complains that the types for

region

don't match. anyone know why?

Because they are different types from TypeScript pov. You’d need to add a type check/convertion.

Your type should be:

export const getSigningKey = (
        secretKey: string | pulumi.Output<string>,
        region: pulumi.Output<digitalocean.Region> | pulumi.Output<aws.Region>
) => {...};

@green-school-95910 thanks, you're right that was an error on my part to not distinguish between either of two outputs and an output that could resolve to either. much appreciated.