don't pass in mutable arguments to CollectFields

instead, have the caller merge the results

spec/Section 6 -- Execution.md

@@ -494,12 +494,9 @@ The depth-first-search order of the field groups produced by {CollectFields()}
 is maintained through execution, ensuring that fields appear in the executed
 response in a stable and predictable order.
 
-CollectFields(objectType, selectionSet, variableValues, groupedFieldSet,
-visitedFragments):
+CollectFields(objectType, selectionSet, variableValues, visitedFragments):
 
-- If {groupedFieldSet} is not provided, initialize it an empty ordered map of
-  lists.
-- If {visitedFragments} is not provided, initialize it to the empty set.
+- Initialize {groupedFieldSet} to an empty ordered map of lists.
 - For each {selection} in {selectionSet}:
   - If {selection} provides the directive `@skip`, let {skipDirective} be that
     directive.
@@ -530,16 +527,31 @@ visitedFragments):
     - If {DoesFragmentTypeApply(objectType, fragmentType)} is false, continue
       with the next {selection} in {selectionSet}.
     - Let {fragmentSelectionSet} be the top-level selection set of {fragment}.
-    - Call {CollectFields(objectType, fragmentSelectionSet, variableValues,
-      groupedFieldSet, visitedFragments)}.
+    - Let {fragmentGroupedFieldSet} be the result of calling
+      {CollectFields(objectType, fragmentSelectionSet, variableValues,
+      visitedFragments)}.
+    - For each {fragmentGroup} in {fragmentGroupedFieldSet}:
+      - Let {responseKey} be the response key shared by all fields in
+        {fragmentGroup}.
+      - Let {groupForResponseKey} be the list in {groupedFieldSet} 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}.
-    - Call {CollectFields(objectType, fragmentSelectionSet, variableValues,
-      groupedFieldSet, visitedFragments)}.
+    - Let {fragmentGroupedFieldSet} be the result of calling
+      {CollectFields(objectType, fragmentSelectionSet, variableValues,
+      visitedFragments)}.
+    - For each {fragmentGroup} in {fragmentGroupedFieldSet}:
+      - Let {responseKey} be the response key shared by all fields in
+        {fragmentGroup}.
+      - Let {groupForResponseKey} be the list in {groupedFieldSet} for
+        {responseKey}; if no such list exists, create it as an empty list.
+      - Append all items in {fragmentGroup} to {groupForResponseKey}.
+- Return {groupedFields} and {visitedFragments}.
 
 DoesFragmentTypeApply(objectType, fragmentType):
 
@@ -562,9 +574,9 @@ Root field collection processes the operation's top-level selection set:
 
 CollectRootFields(rootType, operationSelectionSet, variableValues):
 
-- Initialize {groupedFieldSet} to an empty ordered map of lists.
-- Call {CollectFields(rootType, operationSelectionSet, variableValues
-  groupedFieldSet)}.
+- Initialize {visitedFragments} to the empty set.
+- Let {groupedFieldSet} be the result of calling {CollectFields(rootType,
+  operationSelectionSet, variableValues, visitedFragments)}.
 - Return {groupedFieldSet}.
 
 ### Object Subfield Collection
@@ -573,12 +585,22 @@ Object subfield collection processes a field's sub-selection sets:
 
 CollectSubfields(objectType, fields, variableValues):
 
+- Initialize {visitedFragments} to the empty set.
 - Initialize {groupedSubfieldSet} to an empty ordered map of lists.
 - For each {field} in {fields}:
   - Let {fieldSelectionSet} be the selection set of {field}.
   - If {fieldSelectionSet} is null or empty, continue to the next field.
-  - Call {CollectFields(objectType, fieldSelectionSet, variableValues,
-    groupedSubfieldSet)}.
+  - Let {fieldGroupedFieldSet} and {fieldVisitedFragments} be the result of
+    calling {CollectFields(objectType, fragmentSelectionSet, variableValues,
+    visitedFragments)}.
+  - For each {fieldGroup} in {fieldGroupedFieldSet}:
+    - Let {responseKey} be the response key shared by all fields in
+      {fragmentGroup}.
+    - Let {groupForResponseKey} be the list in {groupedFieldSet} for
+      {responseKey}; if no such list exists, create it as an empty list.
+    - Append all items in {fieldGroup} to {groupForResponseKey}.
+  - For each {fragmentSpreadName} in {fieldVisitedFragments}:
+    - Add {fragmentSpreadName} to {visitedFragments}.
 - Return {groupedSubfieldSet}.
 
 ## Executing Fields