[Fusion] Excluded overridden fields when merging (#8298)

Author: glen-84

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/Extensions/OutputFieldInfoExtensions.cs

@@ -0,0 +1,22 @@
+using System.Collections.Immutable;
+using HotChocolate.Fusion.Info;
+using static HotChocolate.Fusion.WellKnownArgumentNames;
+using static HotChocolate.Fusion.WellKnownDirectiveNames;
+
+namespace HotChocolate.Fusion.Extensions;
+
+internal static class OutputFieldInfoExtensions
+{
+    public static bool IsOverridden(
+        this OutputFieldInfo fieldInfo,
+        ImmutableArray<OutputFieldInfo> fieldGroup)
+    {
+        var overriddenInSchemaNames =
+            fieldGroup
+                .Where(i => i.Field.Directives.ContainsName(Override))
+                .Select(i => (string)i.Field.Directives[Override].First().Arguments[From].Value!)
+                .ToImmutableArray();
+
+        return overriddenInSchemaNames.Contains(fieldInfo.Schema.Name);
+    }
+}

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/SourceSchemaMerger.cs

@@ -531,8 +531,12 @@ .. typeGroup.Where(
         ImmutableArray<OutputFieldInfo> fieldGroup,
         MutableSchemaDefinition mergedSchema)
     {
-        // Filter out all fields marked with @internal.
-        fieldGroup = [.. fieldGroup.Where(i => !i.Field.HasInternalDirective())];
+        // Filter out internal or overridden fields.
+        var group = fieldGroup;
+        fieldGroup =
+        [
+            .. fieldGroup.Where(i => !i.Field.HasInternalDirective() && !i.IsOverridden(group))
+        ];
 
         if (fieldGroup.Length == 0)
         {

src/HotChocolate/Fusion-vnext/test/Fusion.Composition.Tests/SourceSchemaMerger.OutputField.Tests.cs

@@ -236,6 +236,50 @@ type Product
                 }
                 """
             },
+            // The field "price" is only available in Schema C, it is overridden elsewhere.
+            {
+                [
+                    """
+                    type Product @key(fields: "id") {
+                        id: ID!
+                        name: String!
+                        price: Float!
+                    }
+                    """,
+                    """
+                    type Product @key(fields: "id") {
+                        id: ID! @external
+                        price: Float! @override(from: "A")
+                        tax: Float!
+                    }
+                    """,
+                    """
+                    type Product @key(fields: "id") {
+                        id: ID! @external
+                        price: Float! @override(from: "B")
+                        tax: Float!
+                    }
+                    """
+                ],
+                """
+                type Product
+                    @fusion__type(schema: A)
+                    @fusion__type(schema: B)
+                    @fusion__type(schema: C) {
+                    id: ID!
+                        @fusion__field(schema: A)
+                        @fusion__field(schema: B, partial: true)
+                        @fusion__field(schema: C, partial: true)
+                    name: String!
+                        @fusion__field(schema: A)
+                    price: Float!
+                        @fusion__field(schema: C)
+                    tax: Float!
+                        @fusion__field(schema: B)
+                        @fusion__field(schema: C)
+                }
+                """
+            },
             // @provides/@external
             {
                 [