fix field merging behavior for fragments on interfaces (#2380)

Co-authored-by: Kevin Fang <kevin@sorare.com>

codegen/testserver/followschema/interfaces.graphql

@@ -5,10 +5,17 @@ extend type Query {
     noShapeTypedNil: Shape @makeTypedNil
     animal: Animal @makeTypedNil
     notAnInterface: BackedByInterface
+    dog: Dog
 }
 
 interface Animal {
     species: String!
+    size: Size!
+}
+
+type Size {
+    height: Int!
+    weight: Int!
 }
 
 type BackedByInterface {
@@ -19,11 +26,13 @@ type BackedByInterface {
 
 type Dog implements Animal {
     species: String!
+    size: Size!
     dogBreed: String!
 }
 
 type Cat implements Animal {
     species: String!
+    size: Size!
     catBreed: String!
 }
 

codegen/testserver/followschema/interfaces_test.go

@@ -253,4 +253,45 @@ func TestInterfaces(t *testing.T) {
 		require.Equal(t, float64(1), resp.Shapes[1].Coordinates.X)
 		require.Equal(t, float64(1), resp.Shapes[1].Coordinates.Y)
 	})
+
+	t.Run("fragment on interface must return merged fields", func(t *testing.T) {
+		resolvers := &Stub{}
+		resolvers.QueryResolver.Dog = func(ctx context.Context) (dog *Dog, err error) {
+			return &Dog{
+				Size: &Size{
+					Height: 100,
+					Weight: 35,
+				},
+			}, nil
+		}
+
+		c := client.New(handler.NewDefaultServer(NewExecutableSchema(Config{Resolvers: resolvers})))
+		var resp struct {
+			Dog struct {
+				Size struct {
+					Height int
+					Weight int
+				}
+			}
+		}
+
+		c.MustPost(`
+			{
+				dog {
+					size {
+						height
+   					}
+					...AnimalWeight
+				}
+			}
+			fragment AnimalWeight on Animal  {
+ 				size {
+					weight
+				}
+            }
+		`, &resp)
+
+		require.Equal(t, 100, resp.Dog.Size.Height)
+		require.Equal(t, 35, resp.Dog.Size.Weight)
+	})
 }