Fix fragment merge issue (#2558)

Author: ghostdogpr

core/src/main/scala/caliban/execution/Field.scala

@@ -46,22 +46,25 @@ case class Field(
   private[caliban] val aliasedName: String =
     if (alias.isEmpty) name else alias.get
 
-  // TODO: Change the name to `allConditionsUnique` in the next minor version
-  private[caliban] def allFieldsUniqueNameAndCondition: Boolean =
-    fields.isEmpty || fields.tail.isEmpty || allConditionsUniqueLazy
-
-  private lazy val allConditionsUniqueLazy: Boolean = {
-    val headCondition = fields.head._condition
-    var rem           = fields.tail
-    var res           = true
-    val nil           = Nil
-    while ((rem ne nil) && res) {
-      val f = rem.head
-      if (f._condition == headCondition)
+  private[caliban] lazy val allFieldsUniqueNameAndCondition: Boolean = {
+    def inner(fields: List[Field]): Boolean = {
+      val headCondition = fields.head._condition
+
+      val seen = new mutable.HashSet[String]
+      seen.add(fields.head.aliasedName)
+
+      var rem = fields.tail
+      while (rem ne Nil) {
+        val f        = rem.head
+        val continue = seen.add(f.aliasedName) && f._condition == headCondition
+        if (!continue) return false
         rem = rem.tail
-      else res = false
+      }
+      true
     }
-    res
+
+    val fields0 = fields
+    fields0.isEmpty || fields0.tail.isEmpty || inner(fields0)
   }
 
   def combine(other: Field): Field =

core/src/test/scala/caliban/execution/FragmentSpec.scala

@@ -200,6 +200,48 @@ object FragmentSpec extends ZIOSpecDefault {
           isSome(anything)
         )
       },
+      test("nested fragment selection on the same type") {
+        import caliban.schema.Schema.auto._
+
+        case class A(valueA: String, valueC: String)
+        case class B(valueB: String)
+        case class Foo(a: A, b: B)
+        case class Query(foo: Foo)
+
+        val api = graphQL(RootResolver(Query(Foo(A("a", "c"), B("b")))))
+
+        val q = gqldoc("""
+            fragment FragA on Query {
+              foo {
+                a {
+                  valueA
+                  valueC
+                }
+              }
+            }
+
+            fragment FragB on Query {
+              foo {
+                a {
+                  valueA
+                }
+                b {
+                  valueB
+                }
+              }
+            }
+
+            query {
+              ...FragA
+              ...FragB
+            }
+            """)
+
+        api.interpreterUnsafe.execute(q).map { res =>
+          val d = res.data.toString
+          assertTrue(d == """{"foo":{"a":{"valueA":"a","valueC":"c"},"b":{"valueB":"b"}}}""")
+        }
+      },
       suite("spec examples")(
         suite("simple fields")(
           test("merge identical fields") {