Note:

The type lives in https://github.com/apollographql/apollo-tooling but this error is specific to using Apollo Server w/ Federation.

• ❗️ Bug?
  This is the right place! 😄

Before reporting a bug, please check for existing or closed issues
first and read the instructions for filing a bug report:

https://github.com/apollographql/apollo-server/blob/master/CONTRIBUTING.md#reporting-bugs

This bug report should include:

• A short, but descriptive title. The title doesn't need "Apollo" in it.
• The package name and version of Apollo showing the problem.

apollo-graphql

• If applicable, the last version of Apollo where the problem did not occur.

As always occurred, tested on 0.4.1

• The expected behavior.

I should be able to type a __resolveReference function as GraphQLReferenceResolver and have it work with GraphQLResolverMap.

• The actual behavior.

TS error as GraphQLReferenceResolver is not a compatible with GraphQLResolverMap under fieldName.

• A simple, runnable reproduction!
  Please make a GitHub repository that anyone can clone and run and see the
  problem. Other great ways to demonstrate a reproduction are using our
  CodeSandbox template (https://codesandbox.io/s/apollo-server), or re-mixing
  our Glitch template (https://glitch.com/~apollo-launchpad).
  -->

Link to code inside of apollo-tooling:

https://github.com/apollographql/apollo-tooling/blob/master/packages/apollo-graphql/src/schema/resolverMap.ts