Reject selection sets on __typename for abstract parent types

src/HotChocolate/Core/src/Validation/ErrorHelper.cs

@@ -170,7 +170,7 @@ public IError FieldDoesNotExist(FieldNode node, IComplexTypeDefinition outputTyp
 
         public IError LeafFieldsCannotHaveSelections(
             FieldNode node,
-            IComplexTypeDefinition declaringType,
+            ITypeDefinition declaringType,
             IType fieldType)
         {
             return ErrorBuilder.New()

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

@@ -102,6 +102,22 @@ private void ValidateField(
     {
         var namedParentType = parentType.NamedType();
 
+        // __typename can be selected on unions and interfaces, which don't have this field,
+        // so we need this special case for it.
+        if (IntrospectionFieldNames.TypeName.Equals(field.Name.Value, StringComparison.Ordinal))
+        {
+            if (field.SelectionSet is not null)
+            {
+                context.ReportError(
+                    context.LeafFieldsCannotHaveSelections(
+                        field,
+                        namedParentType,
+                        new NonNullType(context.Schema.Types["String"])));
+            }
+
+            return;
+        }
+
         if (namedParentType is not IComplexTypeDefinition complex
             || !complex.Fields.TryGetField(field.Name.Value, out var fieldDef))
         {

src/HotChocolate/Core/test/Validation.Tests/LeafFieldSelectionsRuleTests.cs

@@ -44,6 +44,48 @@ public void Validate_Should_RejectInvalidSelections_WhenEnabled(
         Assert.Equal(expectedMessage, Assert.Single(context.Errors).Message);
     }
 
+    [Theory]
+    [InlineData("{ dog { __typename { name } } }")]
+    [InlineData("{ human { pets { __typename { name } } } }")]
+    [InlineData("{ catOrDog { __typename { name } } }")]
+    [InlineData("{ catOrDog { ...f } } fragment f on CatOrDog { __typename { name } }")]
+    public void Validate_Should_RejectSelectionsOnTypeName_When_ParentIsAnyCompositeType(string sourceText)
+    {
+        // arrange
+        var rule = new LeafFieldSelectionsRule();
+        var document = Utf8GraphQLParser.Parse(sourceText);
+        var context = ValidationUtils.CreateContext(document);
+
+        // act
+        rule.Validate(context, document);
+
+        // assert
+        Assert.Equal(
+            "Field \"__typename\" must not have a selection since type \"String!\" has no subfields.",
+            Assert.Single(context.Errors).Message);
+    }
+
+    [Fact]
+    public void TypeNameOnAbstractTypes()
+    {
+        ExpectValid(
+            """
+            {
+              dog {
+                __typename
+              }
+              human {
+                pets {
+                  __typename
+                }
+              }
+              catOrDog {
+                __typename
+              }
+            }
+            """);
+    }
+
     [Fact]
     public void ScalarSelection()
     {