buildServiceDefinition ignores resolvers for unions

[checkmatez]

When instantiating apollo-server with modules config option, where typedefs contain union type, __resolveType function never gets called, resulting in error:

Abstract type Hi must resolve to an Object type at runtime for field Query.hello with value { there: "Hey!" }, received "undefined". Either the Hi type should provide a "resolveType" function or each possible type should provide an "isTypeOf" function.

Here is reproduction on codesandbox:

[JakeDawkins]

I don't believe this is related to apollo-tooling. I'd suggest asking questions about this on spectrum and seeing if it's an issue before filing an issue on apollo-server

[checkmatez]

I feel it is related to apollo-tooloing, and probably fixed in #1330. Can't confirm right now, will wait for the next release with this fix.

[JakeDawkins]

Ah, I see. Let me know if we need to reopen this

[justinmchase]

I just came to report this, the bug still exists in the latest version.

In fact after debugging, the error is right here:
https://github.com/apollographql/apollo-tooling/blob/master/packages/apollo-tools/src/buildServiceDefinition.ts#L216

It is trying to see if the type is an object type before supporting adding the __ functions (which includes __resolveType) but the union is not reported as an object type. It should probably have a special check for union types, or just remove this limitation and always allow it?

I'm happy to make a PR for this as its blocking me right now. I'm trying to upgrade to use modules but this is affecting several union types I have.