Migrated tests from GraphQL-JS regarding field merging

src/HotChocolate/Core/src/Validation/DocumentValidatorContext.cs

@@ -61,6 +61,9 @@ public IOutputType NonNullString
         public IDictionary<SelectionSetNode, IList<FieldInfo>> FieldSets { get; } =
             new Dictionary<SelectionSetNode, IList<FieldInfo>>();
 
+        public ISet<(FieldNode, FieldNode)> FieldTuples { get; } = 
+            new HashSet<(FieldNode, FieldNode)>();
+
         public ISet<string> VisitedFragments { get; } = new HashSet<string>();
 
         public IDictionary<string, object> VariableValues { get; } =
@@ -130,6 +133,7 @@ public void Clear()
             Path.Clear();
             SelectionSets.Clear();
             FieldSets.Clear();
+            FieldTuples.Clear();
             VisitedFragments.Clear();
             VariableValues.Clear();
             Variables.Clear();

src/HotChocolate/Core/src/Validation/IDocumentValidatorContext.cs

@@ -35,6 +35,11 @@ public interface IDocumentValidatorContext : ISyntaxVisitorContext
         /// A dictionary to store field infos per selection set.
         /// </summary>
         IDictionary<SelectionSetNode, IList<FieldInfo>> FieldSets { get; }
+
+        /// <summary>
+        /// A set of field tuples.
+        /// </summary>
+        ISet<(FieldNode, FieldNode)> FieldTuples { get; }
 
         /// <summary>
         /// Gets a set of already visited fragment names.

src/HotChocolate/Core/src/Validation/Rules/FieldVisitor.cs

@@ -42,7 +42,10 @@ internal sealed class FieldVisitor : TypeDocumentValidatorVisitor
         {
             if (context.FieldSets.Count > 0)
             {
-                TryMergeFieldsInSet(context, context.FieldSets[node.SelectionSet]);
+                foreach (SelectionSetNode selectionSet in context.FieldSets.Keys)
+                {
+                    TryMergeFieldsInSet(context, context.FieldSets[selectionSet]);
+                }
             }
 
             if (node.SelectionSet.Selections.Count == 0)
@@ -75,7 +78,7 @@ internal sealed class FieldVisitor : TypeDocumentValidatorVisitor
             if (context.Types.TryPeek(out IType type) &&
                 type.NamedType() is IComplexOutputType ct)
             {
-                if (ct.Fields.TryGetField(node.Name.Value, out IOutputField of))
+                if (ct.Fields.TryGetField(node.Name.Value, out IOutputField? of))
                 {
                     fields.Add(new FieldInfo(context.Types.Peek(), of.Type, node));
 
@@ -155,6 +158,7 @@ internal sealed class FieldVisitor : TypeDocumentValidatorVisitor
                     context.SelectionSets.Pop();
                 }
             }
+
             return Continue;
         }
 
@@ -235,14 +239,14 @@ private static bool IsTypeNameField(NameString fieldName)
                                     {
                                         TryMergeFieldsInSet(context, fieldA, fieldB);
                                     }
-                                    else
+                                    else if(context.FieldTuples.Add((fieldA.Field, fieldB.Field)))
                                     {
                                         context.Errors.Add(
                                             context.FieldsAreNotMergable(fieldA, fieldB));
                                     }
                                 }
                             }
-                            else
+                            else if(context.FieldTuples.Add((fieldA.Field, fieldB.Field)))
                             {
                                 context.Errors.Add(context.FieldsAreNotMergable(fieldA, fieldB));
                             }