Significant additional changes

- Rewrites default value circular reference checking as "detectDefaultValueCycle()"
- Adds test suite for default value circular references
- Moves default value validation to utility "validateInputValue()"
- Adds "uncoerceDefaultValue()" to preserve behavior of existing programmatically provided default values
- Rewrites "astFromValue()" to remove "uncoerce" and type checking behavior. It used to operate on "internal" coerced values, now it operates on assumed uncoerced values. (also re-writes a bunch of its test suite).
- Extracts "validateInputValue()" from "coerceInputValue()" so it can be used separately

src/index.d.ts

@@ -407,6 +407,8 @@ export {
   visitWithTypeInfo,
   // Coerces a JavaScript value to a GraphQL type, or produces errors.
   coerceInputValue,
+  // Validate a JavaScript value with a GraphQL type, collecting all errors.
+  validateInputValue,
   // Concatenates multiple AST together.
   concatAST,
   // Separates an AST into an AST per Operation.

src/index.js

@@ -396,6 +396,8 @@ export {
   visitWithTypeInfo,
   // Coerces a JavaScript value to a GraphQL type, or produces errors.
   coerceInputValue,
+  // Validate a JavaScript value with a GraphQL type, collecting all errors.
+  validateInputValue,
   // Concatenates multiple AST together.
   concatAST,
   // Separates an AST into an AST per Operation.

src/type/__tests__/validation-test.js

@@ -867,6 +867,104 @@ describe('Type System: Input Objects must have fields', () => {
     ]);
   });
 
+  it('rejects Input Objects with default value circular reference', () => {
+    const validSchema = buildSchema(`
+      type Query {
+        field(arg1: A, arg2: B): String
+      }
+
+      input A {
+        x: A = null
+        y: A = { x: null, y: null }
+        z: [A] = []
+      }
+
+      input B {
+        x: B2 = {}
+      }
+
+      input B2 {
+        x: B3 = {}
+      }
+
+      input B3 {
+        x: B = { x: { x: null } }
+      }
+    `);
+
+    expect(validateSchema(validSchema)).to.deep.equal([]);
+
+    const invalidSchema = buildSchema(`
+      type Query {
+        field(arg1: A, arg2: B, arg3: C, arg4: D, arg5: E): String
+      }
+
+      input A {
+        x: A = {}
+      }
+
+      input B {
+        x: B2 = {}
+      }
+
+      input B2 {
+        x: B3 = {}
+      }
+
+      input B3 {
+        x: B = {}
+      }
+
+      input C {
+        x: [C] = [{}]
+      }
+
+      input D {
+        x: D = { x: { x: {} } }
+      }
+
+      input E {
+        x: E = { x: null }
+        y: E = { y: null }
+      }
+    `);
+
+    expect(validateSchema(invalidSchema)).to.deep.equal([
+      {
+        message:
+          'Cannot reference Input Object field A.x within itself through a series of default values: A.x.',
+        locations: [{ line: 7, column: 16 }],
+      },
+      {
+        message:
+          'Cannot reference Input Object field B.x within itself through a series of default values: B.x, B2.x, B3.x.',
+        locations: [
+          { line: 11, column: 17 },
+          { line: 15, column: 17 },
+          { line: 19, column: 16 },
+        ],
+      },
+      {
+        message:
+          'Cannot reference Input Object field C.x within itself through a series of default values: C.x.',
+        locations: [{ line: 23, column: 18 }],
+      },
+      {
+        message:
+          'Cannot reference Input Object field D.x within itself through a series of default values: D.x.',
+        locations: [{ line: 27, column: 16 }],
+      },
+      {
+        message:
+          'Cannot reference Input Object field E.x within itself through a series of default values: E.x, E.y.',
+        locations: [
+          { line: 31, column: 16 },
+          { line: 32, column: 16 },
+        ],
+      },
+    ]);
+  });
+
   it('rejects an Input Object type with incorrectly typed fields', () => {
     const schema = buildSchema(`
       type Query {

src/type/definition.js

@@ -13,6 +13,7 @@ import { devAssert } from '../jsutils/devAssert';
 import { keyValMap } from '../jsutils/keyValMap';
 import { instanceOf } from '../jsutils/instanceOf';
 import { didYouMean } from '../jsutils/didYouMean';
+import { isIterableObject } from '../jsutils/isIterableObject';
 import { isObjectLike } from '../jsutils/isObjectLike';
 import { identityFunc } from '../jsutils/identityFunc';
 import { suggestionList } from '../jsutils/suggestionList';
@@ -813,7 +814,10 @@ function defineFieldMap<TSource, TContext>(
       name: argName,
       description: argConfig.description,
       type: argConfig.type,
-      defaultValue: argConfig.defaultValue,
+      defaultValue: uncoerceDefaultValue(
+        argConfig.defaultValue,
+        argConfig.type,
+      ),
       deprecationReason: argConfig.deprecationReason,
       extensions: argConfig.extensions && toObjMap(argConfig.extensions),
       astNode: argConfig.astNode,
@@ -1479,7 +1483,10 @@ export class GraphQLInputObjectType {
 
   getFields(): GraphQLInputFieldMap {
     if (typeof this._fields === 'function') {
-      this._fields = this._fields();
+      const _fields = this._fields;
+      // Assign before call to avoid potential infinite recursion.
+      this._fields = {};
+      this._fields = _fields();
     }
     return this._fields;
   }
@@ -1535,7 +1542,10 @@ function defineInputFieldMap(
       name: fieldName,
       description: fieldConfig.description,
       type: fieldConfig.type,
-      defaultValue: fieldConfig.defaultValue,
+      defaultValue: uncoerceDefaultValue(
+        fieldConfig.defaultValue,
+        fieldConfig.type,
+      ),
       deprecationReason: fieldConfig.deprecationReason,
       extensions: fieldConfig.extensions && toObjMap(fieldConfig.extensions),
       astNode: fieldConfig.astNode,
@@ -1587,3 +1597,61 @@ export function isRequiredInputField(
 }
 
 export type GraphQLInputFieldMap = ObjMap<GraphQLInputField>;
+
+/**
+ * Historically GraphQL.js allowed default values to be provided as
+ * assumed-coerced "internal" values, however default values should be provided
+ * as "external" pre-coerced values. `uncoerceDefaultValue()` will convert such
+ * "internal" values to "external" values to avoid breaking existing clients.
+ *
+ * This performs the opposite of `coerceInputValue()`. Given an "internal"
+ * coerced value, reverse the process to provide an "external" uncoerced value.
+ *
+ * This function should not throw Errors on incorrectly shaped input. Instead
+ * it will simply pass through such values directly.
+ *
+ */
+function uncoerceDefaultValue(value: mixed, type: GraphQLInputType): mixed {
+  // Explicitly return the value null.
+  if (value === null) {
+    return null;
+  }
+
+  // Unwrap type
+  const namedType = getNamedType(type);
+
+  if (isIterableObject(value)) {
+    return Array.from(value, (itemValue) =>
+      uncoerceDefaultValue(itemValue, namedType),
+    );
+  }
+
+  if (isInputObjectType(namedType)) {
+    if (!isObjectLike(value)) {
+      return value;
+    }
+
+    const fieldDefs = namedType.getFields();
+    return mapValue(value, (fieldValue, fieldName) =>
+      fieldName in fieldDefs
+        ? uncoerceDefaultValue(fieldValue, fieldDefs[fieldName].type)
+        : fieldValue,
+    );
+  }
+
+  if (isLeafType(namedType)) {
+    try {
+      // For leaf types (Scalars, Enums), serialize is the oppose of coercion
+      // (parseValue) and will produce an "external" value.
+      return namedType.serialize(value);
+    } catch (error) {
+      // Ingore any invalid data errors.
+      // istanbul ignore next - serialize should only throw GraphQLError
+      if (!(error instanceof GraphQLError)) {
+        throw error;
+      }
+    }
+  }
+
+  return value;
+}

src/type/introspection.js

@@ -384,7 +384,10 @@ export const __InputValue: GraphQLObjectType = new GraphQLObjectType({
           'A GraphQL-formatted string representing the default value for this input value.',
         resolve(inputValue) {
           const { type, defaultValue } = inputValue;
-          const valueAST = astFromValue(defaultValue, type);
+          const valueAST =
+            defaultValue !== undefined
+              ? astFromValue(defaultValue, type)
+              : undefined;
           return valueAST ? print(valueAST) : null;
         },
       },