Can not select field from another subgraph on Viewer type returned in mutation payload

[tobias-tengler]

Product

Hot Chocolate

Version

15.0.1

Link to minimal reproduction

b837653

Steps to reproduce

Write a mutation that returns the viewer and selects fields on the viewer that are not available on the subgraph the mutation is on.

mutation {
  doSomething {
    viewer {
      somethingFromAnotherSubgraph
    }
  }
}

What is expected?

Similar to the global object identification specification, the viewer pattern is a well established pattern in the GraphQL world.
To fetch additional fields on the viewer the query planner should check if there is a Query.viewer field on the targeted subgraph and use that to resolve the additional fields on the Viewer type.

What is actually happening?

The query plan generation fails with the error There must be a field resolver definition in this context!.

Relevant log output

Additional context

No response