Field extensions: spec edits

spec/Section 3 -- Type System.md

@@ -1089,20 +1089,41 @@ In this example, a directive is added to a `User` type without adding fields:
 extend type User @addedDirective
 ```
 
+Object type extensions may choose to extend existing fields.
+
+In this example, we can deprecate the field `id` on type `Query` by adding a
+`@deprecated` directive through a type extension:
+
+```graphql example
+type Query {
+  id: String
+}
+```
+
+```graphql example
+extend type Query {
+  id: String @deprecated(reason: "Use newId")
+}
+```
+
 **Type Validation**
 
 Object type extensions have the potential to be invalid if incorrectly defined.
 
 1. The named type must already be defined and must be an Object type.
-2. The fields of an Object type extension must have unique names; no two fields
-   may share the same name.
-3. Any fields of an Object type extension must not be already defined on the
-   previous Object type.
-4. Any non-repeatable directives provided must not already apply to the previous
+2. For each field of an Object type extension:
+   1. The field must have a unique name within that Object type extension; no
+      two fields may share the same name.
+   2. If a field with the same name exists on the previous Object type:
+      1. The field type must match the previous definition exactly.
+      2. The argument definitions defaultValues must match.
+      3. The description or deprecation may be added if none exists, or must
+         match the existing description or deprecation exactly.
+      4. Directives may be added but cannot conflict with existing
+         non-repeatable directives.
+3. Any interfaces provided must not be already implemented by the previous
    Object type.
-5. Any interfaces provided must not be already implemented by the previous
-   Object type.
-6. The resulting extended object type must be a super-set of all interfaces it
+4. The resulting extended object type must be a super-set of all interfaces it
    implements.
 
 ## Interfaces
@@ -1358,6 +1379,21 @@ defined.
 6. The resulting extended Interface type must be a super-set of all Interfaces
    it implements.
 
+#### Interface Field Extensions
+
+Interface field extensions follow the same rules as Object field extensions and
+are used to extend fields on interface types.
+
+```graphql example
+interface Node {
+  id: ID
+}
+
+extend interface Node {
+  id: ID @deprecated(reason: "Use globalId instead")
+}
+```
+
 ## Unions
 
 UnionTypeDefinition : Description? union Name Directives[Const]?