MicroProfile Client ignores NULL ResponseExceptionMapper

Default behavior:

1. Building an MP REST client
2. Request a resource that returns e.g. HTTP 503 status code (MP-Health endpoint which is down)
3. A WebApplicationException is thrown

Alternate behavior:

1. Building an MP REST client
2. Register ResponseExceptionMapper which handles HTTP 503, but returns NULL for org.eclipse.microprofile.rest.client.ext.ResponseExceptionMapper#toThrowable
3. Request a resource which returns HTTP 503
4. A java.lang.reflect.UndeclaredThrowableException is thrown

Expected:

If I interpret it correctly, there should be no exception at all, according to https://github.com/eclipse/microprofile-rest-client/blob/1.4.1/spec/src/main/asciidoc/providers.asciidoc#how-to-implement-responseexceptionmapper.

This method may return null if no throwable should be raised.

Eventual source of evil:

org.jboss.resteasy.microprofile.client.ExceptionMapping.HandlerException

Some code:

public class Ignore503ExceptionMapper implements ResponseExceptionMapper<WebApplicationException> {

    @Override
    public WebApplicationException toThrowable(Response response) {
        return null;
    }

    @Override
    public boolean handles(int status, MultivaluedMap<String, Object> headers) {
        // as per MP-Health specification
        return 503 == status;
    }
}

public interface HealthService extends Closeable {
@GET
@Produces({ MediaType.APPLICATION_JSON})
@Path("/health")
HealthCheckData getHealthData();
}

RestClientBuilder.newBuilder()
.register(new Ignore503ExceptionMapper())
.build(HealthService.class)
.getHealthData();