Validate oneof objects at execution time

This ensures the Oneof Objects resolve to an object with exactly one
non-null entry.

src/execution/__tests__/oneof-test.ts

@@ -37,6 +37,20 @@ function executeQuery(
   return execute({ schema, document: parse(query), rootValue, variableValues });
 }
 
+async function executeQueryAsync(
+  query: string,
+  rootValue: unknown,
+  variableValues?: { [variable: string]: unknown },
+): Promise<ExecutionResult> {
+  const result = await execute({
+    schema,
+    document: parse(query),
+    rootValue,
+    variableValues,
+  });
+  return result;
+}
+
 describe('Execute: Handles Oneof Input Objects and Oneof Objects', () => {
   describe('Oneof Input Objects', () => {
     const rootValue = {
@@ -140,4 +154,123 @@ describe('Execute: Handles Oneof Input Objects and Oneof Objects', () => {
       });
     });
   });
+
+  describe('Oneof Objects', () => {
+    const query = `
+        query ($input: TestInputObject! = {a: "abc"}) {
+          test(input: $input) {
+            a
+            b
+          }
+        }
+      `;
+
+    it('works with a single, non-null value', () => {
+      const rootValue = {
+        test: {
+          a: null,
+          b: 123,
+        },
+      };
+      const result = executeQuery(query, rootValue);
+
+      expectJSON(result).toDeepEqual({
+        data: {
+          test: {
+            a: null,
+            b: 123,
+          },
+        },
+      });
+    });
+
+    it('works with a single, non-null, async value', async () => {
+      const rootValue = {
+        test() {
+          return {
+            a: null,
+            b: () => new Promise((resolve) => resolve(123)),
+          };
+        },
+      };
+      const result = await executeQueryAsync(query, rootValue);
+
+      expectJSON(result).toDeepEqual({
+        data: {
+          test: {
+            a: null,
+            b: 123,
+          },
+        },
+      });
+    });
+
+    it('errors when there are no non-null values', () => {
+      const rootValue = {
+        test: {
+          a: null,
+          b: null,
+        },
+      };
+      const result = executeQuery(query, rootValue);
+
+      expectJSON(result).toDeepEqual({
+        data: { test: null },
+        errors: [
+          {
+            locations: [{ column: 11, line: 3 }],
+            message:
+              'Oneof Object "TestObject" must have exactly one non-null field but got 0.',
+            path: ['test'],
+          },
+        ],
+      });
+    });
+
+    it('errors when there are multiple non-null values', () => {
+      const rootValue = {
+        test: {
+          a: 'abc',
+          b: 456,
+        },
+      };
+      const result = executeQuery(query, rootValue);
+
+      expectJSON(result).toDeepEqual({
+        data: { test: null },
+        errors: [
+          {
+            locations: [{ column: 11, line: 3 }],
+            message:
+              'Oneof Object "TestObject" must have exactly one non-null field but got 2.',
+            path: ['test'],
+          },
+        ],
+      });
+    });
+
+    it('errors when there are multiple non-null, async values', async () => {
+      const rootValue = {
+        test() {
+          return {
+            a: 'abc',
+            b: () => new Promise((resolve) => resolve(123)),
+          };
+        },
+      };
+      const result = await executeQueryAsync(query, rootValue);
+
+      expectJSON(result).toDeepEqual({
+        data: { test: null },
+        errors: [
+          {
+            locations: [{ column: 11, line: 3 }],
+            message:
+              'Oneof Object "TestObject" must have exactly one non-null field but got 2.',
+            path: ['test'],
+          },
+        ],
+      });
+    });
+  });
 });

src/execution/execute.ts

@@ -927,12 +927,13 @@ function completeObjectValue(
         if (!resolvedIsTypeOf) {
           throw invalidReturnTypeError(returnType, result, fieldNodes);
         }
-        return executeFields(
+        return executeFieldsWithOneOfValidation(
           exeContext,
           returnType,
           result,
           path,
           subFieldNodes,
+          fieldNodes,
         );
       });
     }
@@ -942,7 +943,65 @@ function completeObjectValue(
     }
   }
 
-  return executeFields(exeContext, returnType, result, path, subFieldNodes);
+  return executeFieldsWithOneOfValidation(
+    exeContext,
+    returnType,
+    result,
+    path,
+    subFieldNodes,
+    fieldNodes,
+  );
+}
+
+function executeFieldsWithOneOfValidation(
+  exeContext: ExecutionContext,
+  parentType: GraphQLObjectType,
+  sourceValue: unknown,
+  path: Path | undefined,
+  fields: Map<string, ReadonlyArray<FieldNode>>,
+  fieldNodes: ReadonlyArray<FieldNode>,
+): PromiseOrValue<ObjMap<unknown>> {
+  const value = executeFields(
+    exeContext,
+    parentType,
+    sourceValue,
+    path,
+    fields,
+  );
+  if (!parentType.isOneOf) {
+    return value;
+  }
+
+  if (isPromise(value)) {
+    return value.then((resolvedValue) => {
+      validateOneOfValue(resolvedValue, parentType, fieldNodes);
+      return resolvedValue;
+    });
+  }
+
+  validateOneOfValue(value, parentType, fieldNodes);
+  return value;
+}
+
+function validateOneOfValue(
+  value: ObjMap<unknown>,
+  returnType: GraphQLObjectType,
+  fieldNodes: ReadonlyArray<FieldNode>,
+): void {
+  let nonNullCount = 0;
+
+  for (const field in value) {
+    if (value[field] !== null) {
+      nonNullCount += 1;
+    }
+  }
+
+  if (nonNullCount !== 1) {
+    throw new GraphQLError(
+      `Oneof Object "${returnType.name}" must have exactly one non-null field but got ${nonNullCount}.`,
+      fieldNodes,
+    );
+  }
 }
 
 function invalidReturnTypeError(