Validate oneOf types

This validates that all fields of oneOf objects are nullable and that
all fields of oneOf input object are nullable and do not have a
default value.

src/type/__tests__/validation-test.ts

@@ -336,7 +336,7 @@ describe('Type System: A Schema must have Object root types', () => {
     ]);
   });
 
-  it('rejects a schema extended with invalid root types', () => {
+  it('rejects a Schema extended with invalid root types', () => {
     let schema = buildSchema(`
       input SomeInputObject {
         test: String
@@ -1663,6 +1663,69 @@ describe('Type System: Input Object fields must have input types', () => {
   });
 });
 
+describe('Type System: OneOf Object fields must be nullable', () => {
+  it('rejects non-nullable fields', () => {
+    const schema = buildSchema(`
+      type Query {
+        test: SomeObject
+      }
+
+      type SomeObject @oneOf {
+        a: String
+        b: String!
+      }
+    `);
+    expectJSON(validateSchema(schema)).toDeepEqual([
+      {
+        message:
+          'Field SomeObject.b must be nullable as it is part of a OneOf Type.',
+        locations: [{ line: 8, column: 12 }],
+      },
+    ]);
+  });
+});
+
+describe('Type System: OneOf Input Object fields must be nullable', () => {
+  it('rejects non-nullable fields', () => {
+    const schema = buildSchema(`
+      type Query {
+        test(arg: SomeInputObject): String
+      }
+
+      input SomeInputObject @oneOf {
+        a: String
+        b: String!
+      }
+    `);
+    expectJSON(validateSchema(schema)).toDeepEqual([
+      {
+        message: 'OneOf input field SomeInputObject.b must be nullable.',
+        locations: [{ line: 8, column: 12 }],
+      },
+    ]);
+  });
+
+  it('rejects fields with default values', () => {
+    const schema = buildSchema(`
+      type Query {
+        test(arg: SomeInputObject): String
+      }
+
+      input SomeInputObject @oneOf {
+        a: String
+        b: String = "foo"
+      }
+    `);
+    expectJSON(validateSchema(schema)).toDeepEqual([
+      {
+        message:
+          'OneOf input field SomeInputObject.b cannot have a default value.',
+        locations: [{ line: 8, column: 9 }],
+      },
+    ]);
+  });
+});
+
 describe('Objects must adhere to Interface they implement', () => {
   it('accepts an Object which implements an Interface', () => {
     const schema = buildSchema(`

src/type/validate.ts

@@ -20,6 +20,7 @@ import { isEqualType, isTypeSubTypeOf } from '../utilities/typeComparators';
 
 import type {
   GraphQLEnumType,
+  GraphQLField,
   GraphQLInputField,
   GraphQLInputObjectType,
   GraphQLInterfaceType,
@@ -308,6 +309,23 @@ function validateFields(
         );
       }
     }
+
+    if (isObjectType(type) && type.isOneOf) {
+      validateOneOfObjectField(type, field, context);
+    }
+  }
+}
+
+function validateOneOfObjectField(
+  type: GraphQLObjectType,
+  field: GraphQLField<unknown, unknown, unknown>,
+  context: SchemaValidationContext,
+): void {
+  if (isNonNullType(field.type)) {
+    context.reportError(
+      `Field ${type.name}.${field.name} must be nullable as it is part of a OneOf Type.`,
+      field.astNode?.type,
+    );
   }
 }
 
@@ -531,6 +549,30 @@ function validateInputFields(
         [getDeprecatedDirectiveNode(field.astNode), field.astNode?.type],
       );
     }
+
+    if (inputObj.isOneOf) {
+      validateOneOfInputObjectField(inputObj, field, context);
+    }
+  }
+}
+
+function validateOneOfInputObjectField(
+  type: GraphQLInputObjectType,
+  field: GraphQLInputField,
+  context: SchemaValidationContext,
+): void {
+  if (isNonNullType(field.type)) {
+    context.reportError(
+      `OneOf input field ${type.name}.${field.name} must be nullable.`,
+      field.astNode?.type,
+    );
+  }
+
+  if (field.defaultValue) {
+    context.reportError(
+      `OneOf input field ${type.name}.${field.name} cannot have a default value.`,
+      field.astNode,
+    );
   }
 }