When you set up you user Application Default Credential with the gcloud SDK and you add a impersonated service account (command line gcloud auth application-default login --impersonate-service-account=<Service Account>), the client library don't know how to handled this authentication. It returns this error message

guillaumeblaquiere@MAC514 azure-devops-gcp % go run .                                         
google: error getting credentials using well-known file (/Users/guillaumeblaquiere/.config/gcloud/application_default_credentials.json): unknown credential type: "impersonated_service_account"

I expect a service account impersonation with my user credentials.