Add validation rules for @is

spec/Section 4 -- Composition.md

@@ -728,6 +728,148 @@ interface Node {
 }
 ```
 
+### Validate Is Directives
+
+#### Is Invalid Syntax
+
+**Error Code**
+
+`IS_INVALID_SYNTAX`
+
+**Formal Specification**
+
+(to be completed)
+
+**Explanatory Text**
+
+The `@is` directive’s `field` argument must be syntactically valid GraphQL. If
+the selection map string is malformed (e.g., missing closing braces, unbalanced
+quotes, invalid tokens), then the schema cannot be composed correctly. In such
+cases, the error `IS_INVALID_SYNTAX` is raised.
+
+**Examples**
+
+In the following example, the `@is` directive’s `field` argument is a valid
+selection map and satisfies the rule.
+
+```graphql example
+type Query {
+  personById(id: ID! @is(field: "id")): Person @lookup
+}
+
+type Person {
+  id: ID!
+  name: String
+}
+```
+
+In the following counter-example, the `@is` directive’s `field` argument has
+invalid syntax because it is missing a closing brace.
+
+```graphql counter-example
+type Query {
+  personById(id: ID! @is(field: "{ id ")): Person @lookup
+}
+
+type Person {
+  id: ID!
+  name: String
+}
+```
+
+#### Is Invalid Field Type
+
+**Error Code**
+
+`IS_INVALID_FIELD_TYPE`
+
+**Formal Specification**
+
+(to be completed)
+
+**Explanatory Text**
+
+When using the `@is` directive, the `field` argument must always be a string
+that describes how the arguments can be mapped from the entity type that the
+lookup field resolves. If the `field` argument is provided as a type other than
+a string (such as an integer, boolean, or enum), the directive usage is invalid
+and will cause schema composition to fail.
+
+**Examples**
+
+In the following example, the `@is` directive’s `field` argument is a valid
+string and satisfies the rule.
+
+```graphql example
+type Query {
+  personById(id: ID! @is(field: "id")): Person @lookup
+}
+
+type Person {
+  id: ID!
+  name: String
+}
+```
+
+Since `field` is set to `123` (an integer) instead of a string, this violates
+the rule and triggers an `IS_INVALID_FIELD_TYPE` error.
+
+```graphql counter-example
+type Query {
+  personById(id: ID! @is(field: 123)): Person @lookup
+}
+
+type Person {
+  id: ID!
+  name: String
+}
+```
+
+#### Is Invalid Usage
+
+**Error Code**
+
+`IS_INVALID_USAGE`
+
+**Formal Specification**
+
+(to be completed)
+
+**Explanatory Text**
+
+When using the `@is` directive, the field declaring the argument must be a
+lookup field (i.e. have the `@lookup` directive applied).
+
+**Examples**
+
+In the following example, the `@is` directive is applied to an argument declared
+on a field with the `@lookup` directive, satisfying the rule.
+
+```graphql example
+type Query {
+  personById(id: ID! @is(field: "id")): Person @lookup
+}
+
+type Person {
+  id: ID!
+  name: String
+}
+```
+
+In the following counter-example, the `@is` directive is applied to an argument
+declared on a field without the `@lookup` directive, violating the rule.
+
+```graphql counter-example
+type Query {
+  personById(id: ID! @is(field: "id")): Person
+}
+
+type Person {
+  id: ID!
+  name: String
+}
+```
+
 ### Validate Key Directives
 
 #### Key Fields Select Invalid Type
@@ -5723,6 +5865,59 @@ type Product @inaccessible {
 }
 ```
 
+### Validate Is Directives
+
+#### Is Invalid Field
+
+**Error Code**
+
+`IS_INVALID_FIELD`
+
+**Formal Specification**
+
+(to be completed)
+
+**Explanatory Text**
+
+Even if the field selection map for `@is(field: "…")` is syntactically valid,
+its contents must also be valid within the composed schema. Fields must exist on
+the parent type for them to be referenced by `@is`. In addition, fields
+referencing unknown fields break the valid usage of `@is`, leading to an
+`IS_INVALID_FIELD` error.
+
+**Examples**
+
+In the following example, the `@is` directive’s `field` argument is a valid
+field selection map and satisfies the rule.
+
+```graphql example
+# Schema A
+type Query {
+  personById(id: ID! @is(field: "id")): Person @lookup
+}
+
+type Person {
+  id: ID!
+  name: String
+}
+```
+
+In this counter-example, the `@is` directive references a field (`unknownField`)
+that does not exist on the return type (`Person`), causing an `IS_INVALID_FIELD`
+error.
+
+```graphql counter-example
+# Schema A
+type Query {
+  personById(id: ID! @is(field: "unknownField")): Person @lookup
+}
+
+type Person {
+  id: ID!
+  name: String
+}
+```
+
 ### Validate Provides Directives
 
 #### Provides Invalid Fields