fix: Properly resolve interface types in RemoteSchema

tools/src/main/scala/caliban/tools/RemoteSchema.scala

@@ -147,11 +147,18 @@ object RemoteSchema {
   private def toInterfaceType(
     definition: Definition.TypeSystemDefinition.TypeDefinition.InterfaceTypeDefinition,
     definitions: List[Definition.TypeSystemDefinition.TypeDefinition]
-  ): __Type =
+  ): __Type = {
+    val implementations = definitions.collect {
+      case t @ ObjectTypeDefinition(description, name, implements, directives, fields)
+          if implements.map(_.name).toSet.contains(definition.name) =>
+        toObjectType(t, definitions)
+    }
+
     __Type(
       kind = __TypeKind.INTERFACE,
       name = Some(definition.name),
       description = definition.description,
+      possibleTypes = Some(implementations),
       fields = (args: __DeprecatedArgs) =>
         if (definition.fields.nonEmpty)
           Some(
@@ -162,6 +169,7 @@ object RemoteSchema {
         else None,
       directives = toDirectives(definition.directives)
     )
+  }
 
   private def toInputValue(
     definition: Definition.TypeSystemDefinition.TypeDefinition.InputValueDefinition,

tools/src/test/scala/caliban/tools/RemoteSchemaSpec.scala

@@ -7,6 +7,8 @@ import caliban.schema._
 import zio._
 import zio.test.Assertion._
 import zio.test._
+import schema.Annotations._
+import caliban.Macros.gqldoc
 
 object RemoteSchemaSpec extends DefaultRunnableSpec {
   sealed trait EnumType  extends Product with Serializable
@@ -57,6 +59,39 @@ object RemoteSchemaSpec extends DefaultRunnableSpec {
         sdl           = api.render
         remoteSDL     = remoteAPI.render
       } yield assert(remoteSDL)(equalTo(sdl))
+    },
+    testM("properly resolves interface types") {
+      @GQLInterface
+      sealed trait Node
+
+      sealed trait Viewer
+      case class User(id: String, email: String) extends Node with Viewer
+      case class Superuser(id: String)           extends Node with Viewer
+
+      case class Queries(
+        whoAmI: Node = User("1", "[email protected]")
+      )
+
+      val api   = graphQL(RootResolver(Queries()))
+      val query = gqldoc("""
+             query {
+               whoAmI {
+                 ...on User {
+                   email
+                 }
+                 ...on Node {
+                   id
+                 }
+               }
+              }""")
+
+      for {
+        introspected <- SchemaLoader.fromCaliban(api).load
+        remoteSchema <- ZIO.fromOption(RemoteSchema.parseRemoteSchema(introspected))
+        remoteAPI    <- ZIO.effectTotal(fromRemoteSchema(remoteSchema))
+        interpreter  <- remoteAPI.interpreter
+        res          <- interpreter.check(query)
+      } yield assert(res)(isUnit)
     }
   )