[Fusion] Added pre-merge validation rule ExternalArgumentDefaultMismatch

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

@@ -3,6 +3,7 @@ namespace HotChocolate.Fusion.Logging;
 public static class LogEntryCodes
 {
     public const string DisallowedInaccessible = "DISALLOWED_INACCESSIBLE";
+    public const string ExternalArgumentDefaultMismatch = "EXTERNAL_ARGUMENT_DEFAULT_MISMATCH";
     public const string ExternalMissingOnBase = "EXTERNAL_MISSING_ON_BASE";
     public const string OutputFieldTypesNotMergeable = "OUTPUT_FIELD_TYPES_NOT_MERGEABLE";
 }

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

@@ -76,6 +76,20 @@ public static LogEntry DisallowedInaccessibleDirectiveArgument(
             new SchemaCoordinate(directiveName, argumentName: argument.Name, ofDirective: true),
             schema: schema);
 
+    public static LogEntry ExternalArgumentDefaultMismatch(
+        string argumentName,
+        string fieldName,
+        string typeName)
+    {
+        var coordinate = new SchemaCoordinate(typeName, fieldName, argumentName);
+
+        return new LogEntry(
+            string.Format(LogEntryHelper_ExternalArgumentDefaultMismatch, coordinate),
+            LogEntryCodes.ExternalArgumentDefaultMismatch,
+            LogSeverity.Error,
+            coordinate);
+    }
+
     public static LogEntry ExternalMissingOnBase(string fieldName, string typeName)
         => new(
             string.Format(

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

@@ -0,0 +1,53 @@
+using System.Collections.Immutable;
+using HotChocolate.Fusion.Events;
+using HotChocolate.Language;
+using static HotChocolate.Fusion.Logging.LogEntryHelper;
+
+namespace HotChocolate.Fusion.PreMergeValidation.Rules;
+
+/// <summary>
+/// This rule ensures that arguments on fields marked as <c>@external</c> have default values
+/// compatible with the corresponding arguments on fields from other source schemas where the field
+/// is defined (non-<c>@external</c>).
+/// </summary>
+/// <seealso href="https://graphql.github.io/composite-schemas-spec/draft/#sec-External-Argument-Default-Mismatch">
+/// Specification
+/// </seealso>
+internal sealed class ExternalArgumentDefaultMismatchRule : IEventHandler<OutputFieldGroupEvent>
+{
+    public void Handle(OutputFieldGroupEvent @event, CompositionContext context)
+    {
+        var (fieldName, fieldGroup, typeName) = @event;
+
+        var externalFields = fieldGroup
+            .Where(i => ValidationHelper.IsExternal(i.Field))
+            .ToImmutableArray();
+
+        if (externalFields.Length == 0)
+        {
+            return;
+        }
+
+        var argumentNames = fieldGroup
+            .SelectMany(i => i.Field.Arguments, (_, arg) => arg.Name)
+            .ToImmutableHashSet();
+
+        foreach (var argumentName in argumentNames)
+        {
+            var arguments = fieldGroup
+                .SelectMany(i => i.Field.Arguments.Where(a => a.Name == argumentName))
+                .ToImmutableArray();
+
+            var defaultValue = arguments[0].DefaultValue;
+
+            foreach (var argument in arguments)
+            {
+                if (!SyntaxComparer.BySyntax.Equals(argument.DefaultValue, defaultValue))
+                {
+                    context.Log.Write(
+                        ExternalArgumentDefaultMismatch(argumentName, fieldName, typeName));
+                }
+            }
+        }
+    }
+}

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

@@ -36,6 +36,9 @@
   <data name="LogEntryHelper_DisallowedInaccessibleDirectiveArgument" xml:space="preserve">
     <value>The argument '{0}' on built-in directive type '{1}' is not accessible.</value>
   </data>
+  <data name="LogEntryHelper_ExternalArgumentDefaultMismatch" xml:space="preserve">
+    <value>The argument with schema coordinate '{0}' has inconsistent default values.</value>
+  </data>
   <data name="LogEntryHelper_ExternalMissingOnBase" xml:space="preserve">
     <value>Field '{0}' on type '{1}' is only declared as external.</value>
   </data>

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

@@ -47,6 +47,7 @@ private CompositionResult<SchemaDefinition> MergeSchemaDefinitions(CompositionCo
     private static readonly List<object> _preMergeValidationRules =
     [
         new DisallowedInaccessibleElementsRule(),
+        new ExternalArgumentDefaultMismatchRule(),
         new ExternalMissingOnBaseRule(),
         new OutputFieldTypesMergeableRule()
     ];