Trying to query output type via fragment doesn't work when output is interface

[DoumanAsh]

I didn't raise issue because I need time to prepare sample.

We basically have mutation that returns interface as output instead of concrete type.
If we use fragment to query it, it seems we just get empty output from router (it happens with router as standalone binary and embedded router)

I wonder if anyone noticed bug with fragments before?
Query works if you remove fragment and just inline its content inside query.

[bnjjj]

Would be happy to have a small reproductible example to be able to fix it. Thanks

[DoumanAsh]

@bnjjj I extracted my code into public space
It is wrapper over apollo-router-core which provides simplified interface with ability to use async-graphql schema inside embedded router https://github.com/DoumanAsh/graphql-router-example

We see the same issue with latest preview of apollo-router as separate binary too

Here is my test example as PR DoumanAsh/graphql-router-example#1

It is present on branch: broken_fragment

Revision of apollo-router-core is 86e77965b536bfded97f6ef9687096eb2c520026

Here is supergraph schema generated by Rover https://github.com/DoumanAsh/graphql-router-example/blob/broken_fragment/tests/supergraph.graphql

Here is UserType interface alongside concrete impementations:
https://github.com/DoumanAsh/graphql-router-example/blob/broken_fragment/tests/user.graphql#L5-L10=
https://github.com/DoumanAsh/graphql-router-example/blob/broken_fragment/tests/user.graphql#L15-L17=

My query:
https://github.com/DoumanAsh/graphql-router-example/blob/broken_fragment/tests/router.rs#L343-L354=

    fragment TypeBase on UserType {
      __typename
      description
    }
    query Query {
      meType {
        ...TypeBase
      }
    }

Its result is empty:

{
  "data": {
    "meType": {
       }
    }
}

But if I inline fragment into query I get proper output:

{
  "data": {
     "meType": {
       "__typename": "RegularUser",
       "description":"regular"
     }
    }
}

[DoumanAsh]

I added log to my subgraph implementation to see what query comes from supergraph:

query Query__user__0{meType{__typename ...on RegularUser{__typename description}...on SuperUser{__typename description}}}

In case of inlined fragment seems to be the same

query Query__user__0{meType{__typename ...on RegularUser{__typename description}...on SuperUser{__typename description}}}

[DoumanAsh]

In case you're in doubt about my code.
My subgraph correctly returns:

{"data":{"meType":{"__typename":"RegularUser","description":"regular"}}}

[DoumanAsh]

Let's continue this discussion over issue #928 since someone already raised it