Error on unreplaced Upload scalar

This fixes #175.

GraphQLUpload.js

@@ -57,7 +57,10 @@ const { GraphQLScalarType } = require('graphql')
 module.exports = new GraphQLScalarType({
   name: 'Upload',
   description: 'The `Upload` scalar type represents a file upload.',
-  parseValue: value => value,
+  parseValue: () =>
+    Promise.reject(
+      new Error('`Upload` scalars must have a corresponding multipart file.')
+    ),
   parseLiteral() {
     throw new Error('‘Upload’ scalar literal unsupported.')
   },

GraphQLUpload.test.js

@@ -0,0 +1,80 @@
+const {
+  GraphQLString,
+  GraphQLSchema,
+  GraphQLObjectType,
+  GraphQLBoolean,
+  execute,
+  parse
+} = require('graphql')
+const t = require('tap')
+const GraphQLUpload = require('./GraphQLUpload')
+const snapshotError = require('./test-helpers/snapshotError')
+
+t.test('Missing entry for upload scalar.', async t => {
+  const schema = new GraphQLSchema({
+    query: new GraphQLObjectType({
+      name: 'Query',
+      fields: {
+        image: {
+          description: 'Uploads an image.',
+          type: GraphQLString,
+          args: {
+            name: {
+              description: 'Image name.',
+              type: GraphQLString
+            }
+          },
+          async resolve(parent, { image }) {
+            t.type(image, Promise, 'Upload must be a Promise')
+            try {
+              await image
+              throw new Error('Upload must throw!')
+            } catch (error) {
+              t.matchSnapshot(snapshotError(error), 'Middleware throws.')
+            }
+
+            return true
+          }
+        }
+      }
+    }),
+    mutation: new GraphQLObjectType({
+      name: 'Mutation',
+      fields: {
+        uploadImage: {
+          description: 'Uploads an image.',
+          type: GraphQLBoolean,
+          args: {
+            image: {
+              description: 'Image file.',
+              type: GraphQLUpload
+            }
+          },
+          async resolve(parent, { image }) {
+            t.type(image, Promise, 'Upload must be a Promise')
+            try {
+              await image
+              throw new Error('Upload must throw!')
+            } catch (error) {
+              t.matchSnapshot(snapshotError(error), 'Middleware throws.')
+            }
+
+            return true
+          }
+        }
+      }
+    })
+  })
+
+  await execute({
+    schema,
+    document: parse(
+      `mutation ($image: Upload!) {
+        uploadImage(image: $image)
+      }`
+    ),
+    rootValue: {},
+    contextValue: {},
+    variableValues: { image: 'this is invalid' }
+  })
+})

tap-snapshots/GraphQLUpload.test.js-TAP.test.js

@@ -0,0 +1,13 @@
+/* IMPORTANT
+ * This snapshot file is auto-generated, but designed for humans.
+ * It should be checked into source control and tracked carefully.
+ * Re-generate by setting TAP_SNAPSHOT=1 and running tests.
+ * Make sure to inspect the output below.  Do not ignore changes!
+ */
+'use strict'
+exports[`GraphQLUpload.test.js TAP Missing entry for upload scalar. > Middleware throws. 1`] = `
+{
+  "name": "Error",
+  "message": "\`Upload\` scalars must have a corresponding multipart file."
+}
+`