introduce modified field types

spec/Section 6 -- Execution.md

@@ -552,9 +552,23 @@ set or coercing a scalar value.
 ExecuteField(objectType, objectValue, fieldType, fields, variableValues):
   * Let {field} be the first entry in {fields}.
   * Let {fieldName} be the field name of {field}.
+  * Let {requiredStatus} be the required status of {field}.
   * Let {argumentValues} be the result of {CoerceArgumentValues(objectType, field, variableValues)}
   * Let {resolvedValue} be {ResolveFieldValue(objectType, objectValue, fieldName, argumentValues)}.
-  * Return the result of {CompleteValue(fieldType, fields, resolvedValue, variableValues)}.
+  * Let {modifiedFieldType} be {ModifiedOutputType(fieldType, requiredStatus)}.
+  * Return the result of {CompleteValue(modifiedFieldType, fields, resolvedValue, variableValues)}.
+
+ModifiedOutputType(outputType, requiredStatus):
+  * If {requiredStatus} is 'required' and {outputType} is not a Non-Nullable type:
+    * Return Non-Null with an inner type of {outputType}.
+  * Otherwise if {requiredStatus} is 'optional':
+    * If {outputType} is not a Non-Nullable type:
+      * Return {outputType}.
+    * Otherwise if {outputType} is a Non-Nullable type:
+      * Let {innerOutputType} be the inner type of {outputType}.
+      * Return {innerOutputType}.
+  * Otherwise:
+    * Return {outputType}.
 
 
 ### Coercing Field Arguments