Relax SameResponseShape algorithm to be compatible with covariant fields

At a glance

• Identifier: #883
• Stage: RFC X / Rejected
• Champion: @IvanGoncharov
• Latest activity: Advanced to RFC 1 on 2025-06-12
• Spec PR: https://github.com/graphql/graphql-spec/pull/883

Spec PR description

At the moment covariance rules for interfaces are incompatible with validation rule for overlapping fragments. For example, if we have schema like this:

interface AddressInterface {
  country_code: String
}

type Port implements AddressInterface {
  country_code: String!
}

type Warehouse implements AddressInterface {
  country_code: String
}

type Query {
  addressInterface: AddressInterface
}

This query is valid:

query {
  addressInterface {
    country_code
  }
}

But if you expand interface field into inline fragments will cause a validation error:

query {
  addressInterface {
    ... on Port {
      country_code
    }
    ... on Warehouse {
      country_code
    }
  }
}

This PR fixes this issue.