[Fusion] Added pre-merge validation rule "InputFieldTypesMergeableRule"

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/Logging/LogEntryCodes.cs

@@ -8,6 +8,7 @@ public static class LogEntryCodes
     public const string ExternalOnInterface = "EXTERNAL_ON_INTERFACE";
     public const string ExternalUnused = "EXTERNAL_UNUSED";
     public const string InputFieldDefaultMismatch = "INPUT_FIELD_DEFAULT_MISMATCH";
+    public const string InputFieldTypesNotMergeable = "INPUT_FIELD_TYPES_NOT_MERGEABLE";
     public const string KeyDirectiveInFieldsArg = "KEY_DIRECTIVE_IN_FIELDS_ARG";
     public const string KeyFieldsHasArgs = "KEY_FIELDS_HAS_ARGS";
     public const string KeyFieldsSelectInvalidType = "KEY_FIELDS_SELECT_INVALID_TYPE";

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/Logging/LogEntryHelper.cs

@@ -187,6 +187,27 @@ public static LogEntry InputFieldDefaultMismatch(
             schemaA);
     }
 
+    public static LogEntry InputFieldTypesNotMergeable(
+        InputFieldDefinition field,
+        string typeName,
+        SchemaDefinition schemaA,
+        SchemaDefinition schemaB)
+    {
+        var coordinate = new SchemaCoordinate(typeName, field.Name);
+
+        return new LogEntry(
+            string.Format(
+                LogEntryHelper_InputFieldTypesNotMergeable,
+                coordinate,
+                schemaA.Name,
+                schemaB.Name),
+            LogEntryCodes.InputFieldTypesNotMergeable,
+            LogSeverity.Error,
+            coordinate,
+            field,
+            schemaA);
+    }
+
     public static LogEntry KeyDirectiveInFieldsArgument(
         string entityTypeName,
         Directive keyDirective,

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/PreMergeValidation/Rules/InputFieldTypesMergeableRule.cs

@@ -0,0 +1,46 @@
+using HotChocolate.Fusion.Events;
+using static HotChocolate.Fusion.Logging.LogEntryHelper;
+
+namespace HotChocolate.Fusion.PreMergeValidation.Rules;
+
+/// <summary>
+/// <para>
+/// The input fields of input objects with the same name must be mergeable. This rule ensures that
+/// input objects with the same name in different source schemas have fields that can be merged
+/// consistently without conflicts.
+/// </para>
+/// <para>
+/// Input fields are considered mergeable when they share the same name and have compatible types.
+/// The compatibility of types is determined by their structure (e.g., lists), excluding
+/// nullability. Mergeable input fields with different nullability are considered mergeable,
+/// and the resulting merged field will be the most permissive of the two.
+/// </para>
+/// </summary>
+/// <seealso href="https://graphql.github.io/composite-schemas-spec/draft/#sec-Input-Field-Types-mergeable">
+/// Specification
+/// </seealso>
+internal sealed class InputFieldTypesMergeableRule : IEventHandler<InputFieldGroupEvent>
+{
+    public void Handle(InputFieldGroupEvent @event, CompositionContext context)
+    {
+        var (_, fieldGroup, typeName) = @event;
+
+        for (var i = 0; i < fieldGroup.Length - 1; i++)
+        {
+            var fieldInfoA = fieldGroup[i];
+            var fieldInfoB = fieldGroup[i + 1];
+            var typeA = fieldInfoA.Field.Type;
+            var typeB = fieldInfoB.Field.Type;
+
+            if (!ValidationHelper.SameTypeShape(typeA, typeB))
+            {
+                context.Log.Write(
+                    InputFieldTypesNotMergeable(
+                        fieldInfoA.Field,
+                        typeName,
+                        fieldInfoA.Schema,
+                        fieldInfoB.Schema));
+            }
+        }
+    }
+}

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/Properties/CompositionResources.resx

@@ -51,6 +51,9 @@
   <data name="LogEntryHelper_InputFieldDefaultMismatch" xml:space="preserve">
     <value>The default value '{0}' of input field '{1}' in schema '{2}' differs from the default value of '{3}' in schema '{4}'.</value>
   </data>
+  <data name="LogEntryHelper_InputFieldTypesNotMergeable" xml:space="preserve">
+    <value>The input field '{0}' has a different type shape in schema '{1}' than it does in schema '{2}'.</value>
+  </data>
   <data name="LogEntryHelper_KeyDirectiveInFieldsArgument" xml:space="preserve">
     <value>A @key directive on type '{0}' in schema '{1}' references field '{2}', which must not include directive applications.</value>
   </data>

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

@@ -52,6 +52,7 @@ private CompositionResult<SchemaDefinition> MergeSchemaDefinitions(CompositionCo
         new ExternalOnInterfaceRule(),
         new ExternalUnusedRule(),
         new InputFieldDefaultMismatchRule(),
+        new InputFieldTypesMergeableRule(),
         new KeyDirectiveInFieldsArgumentRule(),
         new KeyFieldsHasArgumentsRule(),
         new KeyFieldsSelectInvalidTypeRule(),

src/HotChocolate/Fusion-vnext/test/Fusion.Composition.Tests/PreMergeValidation/Rules/InputFieldTypesMergeableRuleTests.cs

@@ -0,0 +1,152 @@
+using HotChocolate.Fusion.Logging;
+using HotChocolate.Fusion.PreMergeValidation;
+using HotChocolate.Fusion.PreMergeValidation.Rules;
+
+namespace HotChocolate.Composition.PreMergeValidation.Rules;
+
+public sealed class InputFieldTypesMergeableRuleTests : CompositionTestBase
+{
+    private readonly PreMergeValidator _preMergeValidator =
+        new([new InputFieldTypesMergeableRule()]);
+
+    [Theory]
+    [MemberData(nameof(ValidExamplesData))]
+    public void Examples_Valid(string[] sdl)
+    {
+        // arrange
+        var context = CreateCompositionContext(sdl);
+
+        // act
+        var result = _preMergeValidator.Validate(context);
+
+        // assert
+        Assert.True(result.IsSuccess);
+        Assert.True(context.Log.IsEmpty);
+    }
+
+    [Theory]
+    [MemberData(nameof(InvalidExamplesData))]
+    public void Examples_Invalid(string[] sdl, string[] errorMessages)
+    {
+        // arrange
+        var context = CreateCompositionContext(sdl);
+
+        // act
+        var result = _preMergeValidator.Validate(context);
+
+        // assert
+        Assert.True(result.IsFailure);
+        Assert.Equal(errorMessages, context.Log.Select(e => e.Message).ToArray());
+        Assert.True(context.Log.All(e => e.Code == "INPUT_FIELD_TYPES_NOT_MERGEABLE"));
+        Assert.True(context.Log.All(e => e.Severity == LogSeverity.Error));
+    }
+
+    public static TheoryData<string[]> ValidExamplesData()
+    {
+        return new TheoryData<string[]>
+        {
+            // In this example, the field "name" in "AuthorInput" has compatible types across source
+            // schemas, making them mergeable.
+            {
+                [
+                    """
+                    input AuthorInput {
+                        name: String!
+                    }
+                    """,
+                    """
+                    input AuthorInput {
+                        name: String
+                    }
+                    """
+                ]
+            },
+            // The following example shows that fields are mergeable if they have different
+            // nullability but the named type is the same and the list structure is the same.
+            {
+                [
+                    """
+                    input AuthorInput {
+                        tags: [String!]
+                    }
+                    """,
+                    """
+                    input AuthorInput {
+                        tags: [String]!
+                    }
+                    """,
+                    """
+                    input AuthorInput {
+                        tags: [String]
+                    }
+                    """
+                ]
+            },
+            // Multiple input fields.
+            {
+                [
+                    """
+                    input AuthorInput {
+                        name: String!
+                        tags: [String!]
+                        birthdate: DateTime
+                    }
+                    """,
+                    """
+                    input AuthorInput {
+                        name: String
+                        tags: [String]!
+                        birthdate: DateTime!
+                    }
+                    """
+                ]
+            },
+        };
+    }
+
+    public static TheoryData<string[], string[]> InvalidExamplesData()
+    {
+        return new TheoryData<string[], string[]>
+        {
+            // In this example, the field "birthdate" on "AuthorInput" is not mergeable as the field
+            // has different named types ("String" and "DateTime") across source schemas.
+            {
+                [
+                    """
+                    input AuthorInput {
+                        birthdate: String!
+                    }
+                    """,
+                    """
+                    input AuthorInput {
+                        birthdate: DateTime!
+                    }
+                    """
+                ],
+                [
+                    "Input field 'AuthorInput.birthdate' has a different type shape in schema " +
+                    "'A' than it does in schema 'B'."
+                ]
+            },
+            // List versus non-list.
+            {
+                [
+                    """
+                    input AuthorInput {
+                        birthdate: String!
+                    }
+                    """,
+                    """
+                    input AuthorInput {
+                        birthdate: [String!]
+                    }
+                    """
+                ],
+                [
+                    "Input field 'AuthorInput.birthdate' has a different type shape in schema " +
+                    "'A' than it does in schema 'B'."
+                ]
+            }
+        };
+    }
+}