Gateway throws error for @provides-directive on interfaces and objects implementing interfaces

[ddubbert]

Since @apollo/gateway version 0.6.12, when using the @provides-directive on an interface or an object type implementing an interface, the gateway throws an error.

Last working version: 0.6.11

Expected behavior: The @provides-directive should work on fields of interfaces and object types, as long as the type has a @key-directive.

Actual behavior: The gateway throws following errors:

// User is an interface

GraphQLSchemaValidationError: [products] Product.producer -> uses the @provides directive but Product.producer returns User!, which is not an Object type. @provides can only be used on Object types with at least one @key.

or

// Producer is an object type implementing User

GraphQLSchemaValidationError: [products] Product.producer -> uses the @provides directive but Product.producer returns Producer!, which is not an Object type. @provides can only be used on Object types with at least one @key.

The stacktrace is as follows:

at ApolloGateway.createSchema (.../node_modules/@apollo/gateway/dist/index.js:92:19)
    at ApolloGateway.<anonymous> (.../node_modules/@apollo/gateway/dist/index.js:81:22)
    at Generator.next (<anonymous>)
    at fulfilled (.../node_modules/@apollo/gateway/dist/index.js:4:58)
    at process._tickCallback (internal/process/next_tick.js:68:7)

Reproduction: https://github.com/ddubbert/gateway-provides-issue

Short Type summary: The corresponding type definitions look like this (unnecessary fields were excluded):

// User-Service types

interface User @key(fields: "id") {
    id: ID!
    member_since: DateTime!
    username: String!
}

type Producer implements User @key(fields: "id") {
    id: ID!
    member_since: DateTime!
    username: String!
}

// Product-Service types when extending Producer
extend type Producer @key(fields: "id") {
  id: ID! @external
  username: String! @external
}

type Product @key(fields: "id"){
    id: ID!
    name: String!
    producer: Producer! @provides(fields: "username")
}

// Product-Service types when extending User
extend interface User @key(fields: "id") {
  id: ID! @external
  username: String! @external
}

type Product @key(fields: "id"){
    id: ID!
    name: String!
    producer: User! @provides(fields: "username")
}

[martijnwalraven]

You're right, these are indeed limitations of the current schema composition and query planning implementations: interfaces do not support @key directives, and you can't use @provides on fields that return an interface type. We definitely intend to support this in the future however.

[ddubbert]

You're right, these are indeed limitations of the current schema composition and query planning implementations: interfaces do not support @key directives, and you can't use @provides on fields that return an interface type. We definitely intend to support this in the future however.

Well, it worked on object-types that were implementing an interface (in 0.6.11 downwards) but stopped working since 0.6.12. What about that? Why did it work before?