[Fusion] Fix issue with RemoveUnreferencedDefinitions and custom scalars

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

@@ -136,7 +136,7 @@ public static List<IDirective> GetPossibleFusionLookupDirectivesById(
 
     public static void RemoveUnreferencedDefinitions(
         this MutableSchemaDefinition schema,
-        HashSet<string> preserveInputTypeNames)
+        ImmutableSortedSet<MutableSchemaDefinition> sourceSchemas)
     {
         var touchedDefinitions = new HashSet<ITypeSystemMember>();
         var backlog = new Stack<ITypeSystemMember>();
@@ -161,6 +161,16 @@ public static void RemoveUnreferencedDefinitions(
             backlog.Push(schema.SubscriptionType);
         }
 
+        var preservedTypeNames = GetPreservedTypeNames(sourceSchemas);
+
+        foreach (var typeName in preservedTypeNames)
+        {
+            if (schema.Types.TryGetType(typeName, out var inputType))
+            {
+                backlog.Push(inputType);
+            }
+        }
+
         while (backlog.TryPop(out var type))
         {
             if (!touchedDefinitions.Add(type))
@@ -199,7 +209,7 @@ public static void RemoveUnreferencedDefinitions(
         var definitionsToRemove = new HashSet<ITypeSystemMember>();
         foreach (var type in schema.Types)
         {
-            if (touchedDefinitions.Contains(type) || preserveInputTypeNames.Contains(type.NamedType().Name))
+            if (touchedDefinitions.Contains(type))
             {
                 continue;
             }
@@ -246,6 +256,38 @@ private static List<IDirective> GetFusionLookupDirectives(
             .ToList();
     }
 
+    /// <summary>
+    /// Returns a list of type names for types that must be preserved in the merged schema
+    /// even if they are not directly referenced.
+    /// </summary>
+    private static HashSet<string> GetPreservedTypeNames(ImmutableSortedSet<MutableSchemaDefinition> sourceSchemas)
+    {
+        var preservedTypeNames = new HashSet<string>();
+
+        foreach (var schema in sourceSchemas)
+        {
+            foreach (var type in schema.Types.OfType<IObjectTypeDefinition>())
+            {
+                foreach (var field in type.Fields)
+                {
+                    foreach (var argument in field.Arguments)
+                    {
+                        var argumentInnerType = argument.Type.InnerType();
+
+                        if ((argument.HasRequireDirective || field.IsLookup)
+                            && argumentInnerType is INameProvider { Name: var name }
+                            && !SpecScalarNames.IsSpecScalar(name))
+                        {
+                            preservedTypeNames.Add(name);
+                        }
+                    }
+                }
+            }
+        }
+
+        return preservedTypeNames;
+    }
+
     private static void InspectComplexType(
         ISchemaDefinition schema,
         IComplexTypeDefinition complexType,

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

@@ -99,7 +99,7 @@ public CompositionResult<MutableSchemaDefinition> Merge()
         // Remove unreferenced definitions.
         if (_options.RemoveUnreferencedDefinitions)
         {
-            mergedSchema.RemoveUnreferencedDefinitions(GetPreservedInputTypeNames());
+            mergedSchema.RemoveUnreferencedDefinitions(_schemas);
         }
 
         // Add Fusion definitions.
@@ -273,37 +273,6 @@ private void AddNodeField(MutableSchemaDefinition mergedSchema)
         }
     }
 
-    /// <summary>
-    /// Returns a list of input type names for types that must be preserved in the merged schema
-    /// even if they are not directly referenced.
-    /// </summary>
-    private HashSet<string> GetPreservedInputTypeNames()
-    {
-        var preservedInputTypeNames = new HashSet<string>();
-
-        foreach (var schema in _schemas)
-        {
-            foreach (var type in schema.Types.OfType<IObjectTypeDefinition>())
-            {
-                foreach (var field in type.Fields)
-                {
-                    foreach (var argument in field.Arguments)
-                    {
-                        var argumentInnerType = argument.Type.InnerType();
-
-                        if (argumentInnerType is IInputObjectTypeDefinition inputObjectType
-                            && (argument.HasRequireDirective || field.IsLookup))
-                        {
-                            preservedInputTypeNames.Add(inputObjectType.Name);
-                        }
-                    }
-                }
-            }
-        }
-
-        return preservedInputTypeNames;
-    }
-
     /// <summary>
     /// Takes two arguments with the same name but possibly differing in type, description, or
     /// default value, and returns a single, unified argument definition.

src/HotChocolate/Fusion-vnext/test/Fusion.Composition.Tests/SourceSchemaMergerTests.cs

@@ -112,6 +112,55 @@ public void Merge_FourNamedSchemas_AddsFusionDefinitions()
         SchemaFormatter.FormatAsString(result.Value).MatchSnapshot(extension: ".graphql");
     }
 
+    [Fact]
+    public void Merge_WithRequireCustomScalar_RetainsScalarType()
+    {
+        // arrange
+        var sourceSchemaTextA =
+            new SourceSchemaText(
+                "A",
+                """
+                type Query {
+                    product: Product
+                }
+
+                type Product {
+                    weight: Int!
+                }
+                """);
+        var sourceSchemaTextB =
+            new SourceSchemaText(
+                "B",
+                """
+                type Product {
+                    deliveryEstimate(
+                        zip: String!
+                        weight: Weight! @require(field: "weight")
+                    ): Int!
+                }
+
+                scalar Weight
+                """);
+        var compositionLog = new CompositionLog();
+        var sourceSchemaParser1 = new SourceSchemaParser(sourceSchemaTextA, compositionLog);
+        var sourceSchemaParser2 = new SourceSchemaParser(sourceSchemaTextB, compositionLog);
+        var schema1 = sourceSchemaParser1.Parse().Value;
+        var schema2 = sourceSchemaParser2.Parse().Value;
+        var schemas =
+            ImmutableSortedSet.Create(
+                new SchemaByNameComparer<MutableSchemaDefinition>(), schema1, schema2);
+        new SourceSchemaEnricher(schema1, schemas).Enrich();
+        new SourceSchemaEnricher(schema2, schemas).Enrich();
+        var merger = new SourceSchemaMerger(schemas);
+
+        // act
+        var result = merger.Merge();
+
+        // assert
+        Assert.True(result.IsSuccess);
+        Assert.True(result.Value.Types.ContainsName("Weight"));
+    }
+
     [Fact]
     public void Merge_WithRequireInputObject_RetainsInputObjectType()
     {
@@ -162,4 +211,58 @@ input ProductDimensionInput @inaccessible {
         Assert.True(result.IsSuccess);
         Assert.True(result.Value.Types.ContainsName("ProductDimensionInput"));
     }
+
+    [Fact]
+    public void Merge_WithRequireInputObject_ThatUsesCustomScalar_RetainsScalarDependency()
+    {
+        // arrange
+        var sourceSchemaTextA =
+            new SourceSchemaText(
+                "A",
+                """
+                type Query {
+                    product: Product
+                }
+
+                type Product {
+                    weight: Int!
+                }
+                """);
+        var sourceSchemaTextB =
+            new SourceSchemaText(
+                "B",
+                """
+                type Product {
+                    deliveryEstimate(
+                        zip: String!
+                        dimension: ProductDimensionInput! @require(field: "{ weight }")
+                    ): Int!
+                }
+
+                input ProductDimensionInput @inaccessible {
+                    coordinate: Position!
+                }
+
+                scalar Position
+                """);
+        var compositionLog = new CompositionLog();
+        var sourceSchemaParser1 = new SourceSchemaParser(sourceSchemaTextA, compositionLog);
+        var sourceSchemaParser2 = new SourceSchemaParser(sourceSchemaTextB, compositionLog);
+        var schema1 = sourceSchemaParser1.Parse().Value;
+        var schema2 = sourceSchemaParser2.Parse().Value;
+        var schemas =
+            ImmutableSortedSet.Create(
+                new SchemaByNameComparer<MutableSchemaDefinition>(), schema1, schema2);
+        new SourceSchemaEnricher(schema1, schemas).Enrich();
+        new SourceSchemaEnricher(schema2, schemas).Enrich();
+        var merger = new SourceSchemaMerger(schemas);
+
+        // act
+        var result = merger.Merge();
+
+        // assert
+        Assert.True(result.IsSuccess);
+        Assert.True(result.Value.Types.ContainsName("ProductDimensionInput"));
+        Assert.True(result.Value.Types.ContainsName("Position"));
+    }
 }