Assert subscription field is not introspection.

src/validation/rules/SingleFieldSubscriptionsRule.js

@@ -6,9 +6,10 @@ import type { OperationDefinitionNode } from '../../language/ast';
 import type { ASTValidationContext } from '../ValidationContext';
 
 /**
- * Subscriptions must only include one field.
+ * Subscriptions must only include a non-introspection field.
  *
- * A GraphQL subscription is valid only if it contains a single root field.
+ * A GraphQL subscription is valid only if it contains a single root field and
+ * that root field is not an introspection field.
  */
 export function SingleFieldSubscriptionsRule(
   context: ASTValidationContext,
@@ -25,6 +26,20 @@ export function SingleFieldSubscriptionsRule(
               node.selectionSet.selections.slice(1),
             ),
           );
+        } else {
+          const selection = node.selectionSet.selections[0];
+          const fieldName = selection[0].name.value;
+          // fieldName represents an introspection field if it starts with `__`
+          if (fieldName[0] === '_' && fieldName[1] === '_') {
+            context.reportError(
+              new GraphQLError(
+                node.name
+                  ? `Subscription "${node.name.value}" must not select an introspection top level field.`
+                  : 'Anonymous Subscription must not select an introspection top level field.',
+                node.selectionSet.selections,
+              ),
+            );
+          }
         }
       }
     },