[RFC] Prevent @skip and @include on root subscription selection set

spec/Section 5 -- Validation.md

@@ -258,16 +258,74 @@ query getName {
 - Let {subscriptionType} be the root Subscription type in {schema}.
 - For each subscription operation definition {subscription} in the document:
   - Let {selectionSet} be the top level selection set on {subscription}.
-  - Let {variableValues} be the empty set.
-  - Let {groupedFieldSet} be the result of {CollectFields(subscriptionType,
-    selectionSet, variableValues)}.
+  - Let {groupedFieldSet} be the result of
+    {CollectSubscriptionFields(subscriptionType, selectionSet)}.
   - {groupedFieldSet} must have exactly one entry, which must not be an
     introspection field.
 
+CollectSubscriptionFields(objectType, selectionSet, visitedFragments):
+
+- If {visitedFragments} is not provided, initialize it to the empty set.
+- Initialize {groupedFields} to an empty ordered map of lists.
+- For each {selection} in {selectionSet}:
+  - {selection} must not provide the `@skip` directive.
+  - {selection} must not provide the `@include` directive.
+  - If {selection} is a {Field}:
+    - Let {responseKey} be the response key of {selection} (the alias if
+      defined, otherwise the field name).
+    - Let {groupForResponseKey} be the list in {groupedFields} for
+      {responseKey}; if no such list exists, create it as an empty list.
+    - Append {selection} to the {groupForResponseKey}.
+  - If {selection} is a {FragmentSpread}:
+    - Let {fragmentSpreadName} be the name of {selection}.
+    - If {fragmentSpreadName} is in {visitedFragments}, continue with the next
+      {selection} in {selectionSet}.
+    - Add {fragmentSpreadName} to {visitedFragments}.
+    - Let {fragment} be the Fragment in the current Document whose name is
+      {fragmentSpreadName}.
+    - If no such {fragment} exists, continue with the next {selection} in
+      {selectionSet}.
+    - Let {fragmentType} be the type condition on {fragment}.
+    - If {DoesFragmentTypeApply(objectType, fragmentType)} is {false}, continue
+      with the next {selection} in {selectionSet}.
+    - Let {fragmentSelectionSet} be the top-level selection set of {fragment}.
+    - Let {fragmentGroupedFieldSet} be the result of calling
+      {CollectSubscriptionFields(objectType, fragmentSelectionSet,
+      visitedFragments)}.
+    - For each {fragmentGroup} in {fragmentGroupedFieldSet}:
+      - Let {responseKey} be the response key shared by all fields in
+        {fragmentGroup}.
+      - Let {groupForResponseKey} be the list in {groupedFields} for
+        {responseKey}; if no such list exists, create it as an empty list.
+      - Append all items in {fragmentGroup} to {groupForResponseKey}.
+  - If {selection} is an {InlineFragment}:
+    - Let {fragmentType} be the type condition on {selection}.
+    - If {fragmentType} is not {null} and {DoesFragmentTypeApply(objectType,
+      fragmentType)} is {false}, continue with the next {selection} in
+      {selectionSet}.
+    - Let {fragmentSelectionSet} be the top-level selection set of {selection}.
+    - Let {fragmentGroupedFieldSet} be the result of calling
+      {CollectSubscriptionFields(objectType, fragmentSelectionSet,
+      visitedFragments)}.
+    - For each {fragmentGroup} in {fragmentGroupedFieldSet}:
+      - Let {responseKey} be the response key shared by all fields in
+        {fragmentGroup}.
+      - Let {groupForResponseKey} be the list in {groupedFields} for
+        {responseKey}; if no such list exists, create it as an empty list.
+      - Append all items in {fragmentGroup} to {groupForResponseKey}.
+- Return {groupedFields}.
+
+Note: This algorithm is very similar to {CollectFields()}, it differs in that it
+does not have access to runtime variables and thus the `@skip` and `@include`
+directives cannot be used.
+
 **Explanatory Text**
 
 Subscription operations must have exactly one root field.
 
+To enable us to determine this without access to runtime variables, we must
+forbid the `@skip` and `@include` directives in the root selection set.
+
 Valid examples:
 
 ```graphql example
@@ -318,6 +376,19 @@ fragment multipleSubscriptions on Subscription {
 }
 ```
 
+We do not allow the `@skip` and `@include` directives at the root of the
+subscription operation. The following example is also invalid:
+
+```graphql counter-example
+subscription requiredRuntimeValidation($bool: Boolean!) {
+  newMessage @include(if: $bool) {
+    body
+    sender
+  }
+  disallowedSecondRootField @skip(if: $bool)
+}
+```
+
 The root field of a subscription operation must not be an introspection field.
 The following example is also invalid: