feat: support the oneOf directive

execution/engine/engine_config_test.go

@@ -399,6 +399,12 @@ directive @specifiedBy(
     url: String!
 ) on SCALAR
 
+"""
+The @oneOf built-in directive is used within the type system definition language
+to indicate an Input Object is a OneOf Input Object.
+"""
+directive @oneOf on INPUT_OBJECT
+
 """
 A Directive provides a way to describe alternate runtime execution and type validation behavior in a GraphQL document.
 In some cases, you need to provide options to alter GraphQL's execution behavior

execution/engine/testdata/full_introspection.json

@@ -735,6 +735,14 @@
               "defaultValue": null
             }
           ]
+        },
+        {
+          "name": "oneOf",
+          "description": "The @oneOf built-in directive is used within the type system definition language\nto indicate an Input Object is a OneOf Input Object.",
+          "locations": [
+            "INPUT_OBJECT"
+          ],
+          "args": []
         }
       ]
     }

execution/engine/testdata/full_introspection_with_deprecated.json

@@ -769,6 +769,14 @@
               "defaultValue": null
             }
           ]
+        },
+        {
+          "name": "oneOf",
+          "description": "The @oneOf built-in directive is used within the type system definition language\nto indicate an Input Object is a OneOf Input Object.",
+          "locations": [
+            "INPUT_OBJECT"
+          ],
+          "args": []
         }
       ]
     }

execution/engine/testdata/full_introspection_with_typenames.json

@@ -831,6 +831,15 @@
               "defaultValue": null
             }
           ]
+        },
+        {
+          "__typename": "__Directive",
+          "name": "oneOf",
+          "description": "The @oneOf built-in directive is used within the type system definition language\nto indicate an Input Object is a OneOf Input Object.",
+          "locations": [
+            "INPUT_OBJECT"
+          ],
+          "args": []
         }
       ]
     }

v2/pkg/asttransform/baseschema.go

@@ -168,6 +168,12 @@ directive @deprecated(
 
 directive @specifiedBy(url: String!) on SCALAR
 
+"""
+The @oneOf built-in directive is used within the type system definition language
+to indicate an Input Object is a OneOf Input Object.
+"""
+directive @oneOf on INPUT_OBJECT
+
 """
 A Directive provides a way to describe alternate runtime execution and type validation behavior in a GraphQL document.
 In some cases, you need to provide options to alter GraphQL's execution behavior

v2/pkg/asttransform/fixtures/complete.golden

@@ -57,6 +57,12 @@ directive @specifiedBy(
     url: String!
 ) on SCALAR
 
+"""
+The @oneOf built-in directive is used within the type system definition language
+to indicate an Input Object is a OneOf Input Object.
+"""
+directive @oneOf on INPUT_OBJECT
+
 """
 A Directive provides a way to describe alternate runtime execution and type validation behavior in a GraphQL document.
 In some cases, you need to provide options to alter GraphQL's execution behavior

v2/pkg/asttransform/fixtures/custom_query_name.golden

@@ -57,6 +57,12 @@ directive @specifiedBy(
     url: String!
 ) on SCALAR
 
+"""
+The @oneOf built-in directive is used within the type system definition language
+to indicate an Input Object is a OneOf Input Object.
+"""
+directive @oneOf on INPUT_OBJECT
+
 """
 A Directive provides a way to describe alternate runtime execution and type validation behavior in a GraphQL document.
 In some cases, you need to provide options to alter GraphQL's execution behavior

v2/pkg/asttransform/fixtures/mutation_only.golden

@@ -49,6 +49,12 @@ directive @specifiedBy(
     url: String!
 ) on SCALAR
 
+"""
+The @oneOf built-in directive is used within the type system definition language
+to indicate an Input Object is a OneOf Input Object.
+"""
+directive @oneOf on INPUT_OBJECT
+
 """
 A Directive provides a way to describe alternate runtime execution and type validation behavior in a GraphQL document.
 In some cases, you need to provide options to alter GraphQL's execution behavior

v2/pkg/asttransform/fixtures/schema_missing.golden

@@ -57,6 +57,12 @@ directive @specifiedBy(
     url: String!
 ) on SCALAR
 
+"""
+The @oneOf built-in directive is used within the type system definition language
+to indicate an Input Object is a OneOf Input Object.
+"""
+directive @oneOf on INPUT_OBJECT
+
 """
 A Directive provides a way to describe alternate runtime execution and type validation behavior in a GraphQL document.
 In some cases, you need to provide options to alter GraphQL's execution behavior

v2/pkg/asttransform/fixtures/simple.golden

@@ -57,6 +57,12 @@ directive @specifiedBy(
     url: String!
 ) on SCALAR
 
+"""
+The @oneOf built-in directive is used within the type system definition language
+to indicate an Input Object is a OneOf Input Object.
+"""
+directive @oneOf on INPUT_OBJECT
+
 """
 A Directive provides a way to describe alternate runtime execution and type validation behavior in a GraphQL document.
 In some cases, you need to provide options to alter GraphQL's execution behavior

v2/pkg/asttransform/fixtures/subscription_only.golden

@@ -48,6 +48,12 @@ directive @specifiedBy(
     url: String!
 ) on SCALAR
 
+"""
+The @oneOf built-in directive is used within the type system definition language
+to indicate an Input Object is a OneOf Input Object.
+"""
+directive @oneOf on INPUT_OBJECT
+
 """
 A Directive provides a way to describe alternate runtime execution and type validation behavior in a GraphQL document.
 In some cases, you need to provide options to alter GraphQL's execution behavior

v2/pkg/asttransform/fixtures/subscription_renamed.golden

@@ -48,6 +48,12 @@ directive @specifiedBy(
     url: String!
 ) on SCALAR
 
+"""
+The @oneOf built-in directive is used within the type system definition language
+to indicate an Input Object is a OneOf Input Object.
+"""
+directive @oneOf on INPUT_OBJECT
+
 """
 A Directive provides a way to describe alternate runtime execution and type validation behavior in a GraphQL document.
 In some cases, you need to provide options to alter GraphQL's execution behavior

v2/pkg/asttransform/fixtures/with_mutation_subscription.golden

@@ -68,6 +68,12 @@ directive @specifiedBy(
     url: String!
 ) on SCALAR
 
+"""
+The @oneOf built-in directive is used within the type system definition language
+to indicate an Input Object is a OneOf Input Object.
+"""
+directive @oneOf on INPUT_OBJECT
+
 """
 A Directive provides a way to describe alternate runtime execution and type validation behavior in a GraphQL document.
 In some cases, you need to provide options to alter GraphQL's execution behavior

v2/pkg/astvalidation/operation_rule_values.go

@@ -430,6 +430,11 @@ func (v *valuesVisitor) valueSatisfiesInputObjectTypeDefinition(value ast.Value,
 		return false
 	}
 
+	// Validate @oneOf constraint if present
+	if v.objectValueViolatesOneOf(value, inputObjectTypeDefinition) {
+		return false
+	}
+
 	return true
 }
 
@@ -464,6 +469,74 @@ func (v *valuesVisitor) objectValueHasDuplicateFields(objectValue int) bool {
 	return hasDuplicates
 }
 
+// objectValueViolatesOneOf checks if an input object value violates the @oneOf directive constraint.
+func (v *valuesVisitor) objectValueViolatesOneOf(objectValue ast.Value, defRef int) bool {
+	def := v.definition.InputObjectTypeDefinitions[defRef]
+	// Check if the input object type has @oneOf directive
+	if !def.HasDirectives {
+		return false
+	}
+	hasOneOfDirective := def.Directives.HasDirectiveByName(v.definition, "oneOf")
+	if !hasOneOfDirective {
+		return false
+	}
+
+	fieldRefs := v.operation.ObjectValues[objectValue.Ref].Refs
+	if len(fieldRefs) != 1 {
+		objName := v.definition.InputObjectTypeDefinitionNameBytes(defRef)
+		v.Report.AddExternalError(operationreport.ErrOneOfInputObjectFieldCount(objName, len(fieldRefs), objectValue.Position))
+		return true
+	}
+
+	type nullableVar struct {
+		fieldRef              int
+		fieldValue            ast.Value
+		variableDefinitionRef int
+	}
+	var nullableVars []nullableVar
+
+	for _, fieldRef := range fieldRefs {
+		fieldValue := v.operation.ObjectFieldValue(fieldRef)
+
+		if fieldValue.Kind == ast.ValueKindNull {
+			objName := v.definition.InputObjectTypeDefinitionNameBytes(defRef)
+			fieldName := v.operation.ObjectFieldNameBytes(fieldRef)
+			v.Report.AddExternalError(operationreport.ErrOneOfInputObjectNullValue(objName, fieldName, fieldValue.Position))
+			return true
+		}
+
+		if fieldValue.Kind == ast.ValueKindVariable {
+			// For variables, check if the variable type is nullable
+			variableDefinitionRef, variableTypeRef, _, ok := v.operationVariableType(fieldValue.Ref)
+			if !ok {
+				continue
+			}
+
+			// Collect nullable variables
+			if v.operation.Types[variableTypeRef].TypeKind != ast.TypeKindNonNull {
+				nullableVars = append(nullableVars, nullableVar{
+					fieldRef,
+					fieldValue,
+					variableDefinitionRef})
+			}
+		}
+	}
+
+	// If exactly one field, but it's a nullable variable, report that error
+	if len(nullableVars) > 0 {
+		violation := nullableVars[0]
+		objName := v.definition.InputObjectTypeDefinitionNameBytes(defRef)
+		fieldName := v.operation.ObjectFieldNameBytes(violation.fieldRef)
+		variableName := v.operation.VariableValueNameBytes(violation.fieldValue.Ref)
+		v.Report.AddExternalError(operationreport.ErrOneOfInputObjectNullableVariable(
+			objName, fieldName, variableName, violation.fieldValue.Position,
+			v.operation.VariableDefinitions[violation.variableDefinitionRef].VariableValue.Position))
+		return true
+	}
+
+	return false
+}
+
 func (v *valuesVisitor) objectFieldDefined(objectField, inputObjectTypeDefinition int) bool {
 	name := v.operation.ObjectFieldNameBytes(objectField)
 	for _, i := range v.definition.InputObjectTypeDefinitions[inputObjectTypeDefinition].InputFieldsDefinition.Refs {