Avoid relying on constructor.name for instanceOf error check.

An improvement beyond #1174, for non-production environments that enable
minification, such as NODE_ENV='staging'.

Since graphql-js goes to the trouble of providing a Symbol.toStringTag
property for most of the classes it exports, and that string is immune to
minification (unlike constructor.name), we should use it for error
checking in instanceOf(value, constructor) whenever the constructor
provides a tag, falling back to constructor.name and
value.constructor.name for constructors that do not define
Symbol.toStringTag (as before).

Motivating issue/investigation:
apollographql/apollo-client#7446 (comment)

src/jsutils/__tests__/instanceOf-test.js

@@ -2,6 +2,15 @@ import { expect } from 'chai';
 import { describe, it } from 'mocha';
 
 import { instanceOf } from '../instanceOf';
+import { SYMBOL_TO_STRING_TAG } from '../../polyfills/symbols';
+import {
+  GraphQLScalarType,
+  GraphQLObjectType,
+  GraphQLInterfaceType,
+  GraphQLUnionType,
+  GraphQLEnumType,
+  GraphQLInputObjectType,
+} from '../../type/definition';
 
 describe('instanceOf', () => {
   it('fails with descriptive error message', () => {
@@ -19,4 +28,106 @@ describe('instanceOf', () => {
       /^Cannot use Foo "\[object Object\]" from another module or realm./m,
     );
   });
+
+  describe('Symbol.toStringTag', () => {
+    function checkSameNameClasses(getClass) {
+      const Class1 = getClass('FirstClass');
+      const Class2 = getClass('SecondClass');
+
+      expect(Class1.name).to.equal('Foo');
+      expect(Class2.name).to.equal('Foo');
+
+      expect(instanceOf(null, Class1)).to.equal(false);
+      expect(instanceOf(null, Class2)).to.equal(false);
+
+      const c1 = new Class1();
+      const c2 = new Class2();
+
+      expect(getTag(c1)).to.equal('FirstClass');
+      expect(getTag(c2)).to.equal('SecondClass');
+
+      // In these Symbol.toStringTag tests, instanceOf returns the
+      // expected boolean value without throwing an error, because even
+      // though Class1.name === Class2.name, the Symbol.toStringTag
+      // strings of the two classes are different.
+      expect(instanceOf(c1, Class1)).to.equal(true);
+      expect(instanceOf(c1, Class2)).to.equal(false);
+      expect(instanceOf(c2, Class1)).to.equal(false);
+      expect(instanceOf(c2, Class2)).to.equal(true);
+    }
+
+    function getTag(from: any): string {
+      return from[SYMBOL_TO_STRING_TAG];
+    }
+
+    it('does not fail if dynamically-defined tags differ', () => {
+      checkSameNameClasses((tag) => {
+        class Foo {}
+        Object.defineProperty(Foo.prototype, SYMBOL_TO_STRING_TAG, {
+          value: tag,
+        });
+        return Foo;
+      });
+    });
+
+    it('does not fail if dynamically-defined tag getters differ', () => {
+      checkSameNameClasses((tag) => {
+        class Foo {}
+        Object.defineProperty(Foo.prototype, SYMBOL_TO_STRING_TAG, {
+          get() {
+            return tag;
+          },
+        });
+        return Foo;
+      });
+    });
+
+    it('does not fail for anonymous classes', () => {
+      checkSameNameClasses((tag) => {
+        const Foo = class {};
+        Object.defineProperty(Foo.prototype, SYMBOL_TO_STRING_TAG, {
+          get() {
+            return tag;
+          },
+        });
+        return Foo;
+      });
+    });
+
+    it('does not fail if prototype property tags differ', () => {
+      checkSameNameClasses((tag) => {
+        class Foo {}
+        (Foo.prototype: any)[SYMBOL_TO_STRING_TAG] = tag;
+        return Foo;
+      });
+    });
+
+    it('does not fail if computed getter tags differ', () => {
+      checkSameNameClasses((tag) => {
+        class Foo {
+          // $FlowFixMe[unsupported-syntax] Flow doesn't support computed properties yet
+          get [SYMBOL_TO_STRING_TAG]() {
+            return tag;
+          }
+        }
+        return Foo;
+      });
+    });
+
+    it('is defined for various GraphQL*Type classes', () => {
+      function checkGraphQLType(constructor, expectedName) {
+        expect(getTag(constructor.prototype)).to.equal(expectedName);
+        const instance = Object.create(constructor.prototype);
+        expect(getTag(instance)).to.equal(expectedName);
+        expect(instanceOf(instance, constructor)).to.equal(true);
+      }
+
+      checkGraphQLType(GraphQLScalarType, 'GraphQLScalarType');
+      checkGraphQLType(GraphQLObjectType, 'GraphQLObjectType');
+      checkGraphQLType(GraphQLInterfaceType, 'GraphQLInterfaceType');
+      checkGraphQLType(GraphQLUnionType, 'GraphQLUnionType');
+      checkGraphQLType(GraphQLEnumType, 'GraphQLEnumType');
+      checkGraphQLType(GraphQLInputObjectType, 'GraphQLInputObjectType');
+    });
+  });
 });

src/jsutils/instanceOf.js

@@ -1,3 +1,5 @@
+import { SYMBOL_TO_STRING_TAG } from '../polyfills/symbols';
+
 /**
  * A replacement for instanceof which includes an error warning when multi-realm
  * constructors are detected.
@@ -15,9 +17,24 @@ export const instanceOf: (mixed, mixed) => boolean =
           return true;
         }
         if (value) {
-          const valueClass = value.constructor;
-          const className = constructor.name;
-          if (className && valueClass && valueClass.name === className) {
+          const proto = constructor && constructor.prototype;
+          const classTag = proto && proto[SYMBOL_TO_STRING_TAG];
+          const className = classTag || constructor.name;
+          // When the constructor class defines a Symbol.toStringTag
+          // property, as most classes exported by graphql-js do, use it
+          // instead of constructor.name and value.constructor.name to
+          // detect module/realm duplication, since the Symbol.toStringTag
+          // string is immune to minification. This code runs only when
+          // process.env.NODE_ENV !== 'production', but minification is
+          // often enabled in non-production environments like 'staging'.
+          // In these environments, this error can be thrown mistakenly if
+          // we rely on constructor.name and value.constructor.name, since
+          // they could be minified to the same short string, even though
+          // value is legitimately _not_ instanceof constructor.
+          const valueName = classTag
+                ? value[SYMBOL_TO_STRING_TAG]
+                : value.constructor && value.constructor.name;
+          if (typeof className === 'string' && valueName === className) {
             throw new Error(
               `Cannot use ${className} "${value}" from another module or realm.