From d448b28f7bc2ab21b8bbedc348f2d1902af8fa18 Mon Sep 17 00:00:00 2001
From: Uma <uma.zalakain@itv.com>
Date: Wed, 10 Apr 2024 08:59:33 +0100
Subject: [PATCH] Fragment applicability by subtype intersection

As outlined in https://spec.graphql.org/October2021/#sec-Fragment-spread-is-possible
---
 modules/core/src/main/scala/compiler.scala | 19 ++++++++-----------
 modules/core/src/main/scala/schema.scala   |  9 ++++++++-
 2 files changed, 16 insertions(+), 12 deletions(-)

diff --git a/modules/core/src/main/scala/compiler.scala b/modules/core/src/main/scala/compiler.scala
index 80241477..6836f044 100644
--- a/modules/core/src/main/scala/compiler.scala
+++ b/modules/core/src/main/scala/compiler.scala
@@ -977,7 +977,7 @@ object QueryCompiler {
               f      <- Elab.fragment(nme)
               ctpe   =  c.tpe.underlyingNamed
               subtpe <- Elab.liftR(s.definition(f.tpnme).toResult(s"Unknown type '${f.tpnme}' in type condition of fragment '$nme'"))
-              _      <- Elab.failure(s"Fragment '$nme' is not compatible with type '${c.tpe}'").whenA(!fragmentApplies(subtpe, ctpe))
+              _      <- Elab.failure(s"Fragment '$nme' is not compatible with type '${c.tpe}'").whenA(!fragmentApplies(s, subtpe, ctpe))
               _      <- Elab.push(c.asType(subtpe), f.child)
               ec     <- transform(f.child)
               _      <- Elab.pop
@@ -999,7 +999,7 @@ object QueryCompiler {
                           case Some(tpnme) =>
                             Elab.liftR(s.definition(tpnme).toResult(s"Unknown type '$tpnme' in type condition inline fragment"))
                         }
-              _      <- Elab.failure(s"Inline fragment with type condition '$subtpe' is not compatible with type '$ctpe'").whenA(!fragmentApplies(subtpe, ctpe))
+              _      <- Elab.failure(s"Inline fragment with type condition '$subtpe' is not compatible with type '$ctpe'").whenA(!fragmentApplies(s, subtpe, ctpe))
               _      <- Elab.push(c.asType(subtpe), child)
               ec     <- transform(child)
               _      <- Elab.pop
@@ -1013,16 +1013,13 @@ object QueryCompiler {
 
     /**
      * Tests the supplied type condition is satisfied by the supplied context type
+     * https://spec.graphql.org/October2021/#sec-Fragment-spread-is-possible
      */
-    def fragmentApplies(typeCond: NamedType, ctpe: NamedType): Boolean =
-      (typeCond.dealias, ctpe.dealias) match {
-        case (u: UnionType, _) =>
-          u.members.exists(m => fragmentApplies(m, ctpe))
-        case (_, u: UnionType) =>
-          u.members.exists(m => fragmentApplies(typeCond, m))
-        case _ =>
-          typeCond <:< ctpe || ctpe <:< typeCond
-      }
+    def fragmentApplies(schema: Schema, typeCond: NamedType, ctpe: NamedType): Boolean = {
+      val typeCondPoss = schema.subtypes(typeCond.dealias)
+      val ctpePoss = schema.subtypes(ctpe.dealias)
+      typeCondPoss.exists(x => ctpePoss.exists(y => x =:= y))
+    }
 
     def elaborateBinding(b: Binding, vars: Vars): Elab[Binding] =
       Elab.liftR(Value.elaborateValue(b.value, vars).map(ev => b.copy(value = ev)))
diff --git a/modules/core/src/main/scala/schema.scala b/modules/core/src/main/scala/schema.scala
index 2b619bc6..dfc4941d 100644
--- a/modules/core/src/main/scala/schema.scala
+++ b/modules/core/src/main/scala/schema.scala
@@ -257,6 +257,9 @@ trait Schema {
       case _ => schemaType
     }
   }
+
+  /** Returns all subtypes of `tpe` */
+  def subtypes(tpe: NamedType): Set[NamedType] = types.filter(_ <:< tpe).toSet
 }
 
 object Schema {
@@ -575,6 +578,8 @@ sealed trait Type extends Product {
 
   def isUnion: Boolean = false
 
+  def isObject: Boolean = false
+
   def /(pathElement: String): Path =
     Path.from(this) / pathElement
 
@@ -784,7 +789,9 @@ case class ObjectType(
   fields: List[Field],
   interfaces: List[NamedType],
   directives: List[Directive]
-) extends TypeWithFields
+) extends TypeWithFields {
+  override def isObject: Boolean = true
+}
 
 /**
  * Object extensions allow additional fields to be added to a pre-existing object type