Adds Non Null Input fields cannot be inaccessible

spec/Section 4 -- Composition.md

@@ -18,4 +18,121 @@ run in sequence to produce the composite execution schema.
 
 ### Post Merge Validation
 
+#### Non-Null Input Fields cannot be inaccessible
+
+**Error Code**
+
+NON_NULL_INPUT_FIELD_IS_INACCESSIBLE
+
+**Formal Specification**
+
+- Let {fields} be the set of all fields of the input types
+- For each {field} in {fields}:
+  - If {field} is a non-null input field:
+    - {field} must not be declared as `@inaccessible`
+
+**Explanatory Text**
+
+In a composed schema, a non-null input field must always be accessible. If it is
+not, the field would require a value, yet the user would be unable to supply
+one.
+
+A valid case where a public input field references another public input type:
+
+```graphql example
+input Input1 {
+  field1: String!
+  field2: Input2
+}
+
+input Input2 {
+  field3: String
+}
+```
+
+Another valid case is where the field is not exposed in the composed schema:
+
+```graphql example
+input Input1 {
+  field1: String!
+  field2: Input2 @inaccessible
+}
+
+input Input2 @inaccessible {
+  field3: String
+}
+```
+
+An invalid case is when a non-null input field is inaccessible:
+
+```graphql counter-example
+input Input1 {
+  field1: String! @inaccessible
+  field2: Input2
+}
+
+input Input2 {
+  field3: String
+}
+```
+
+#### Input Fields cannot reference inaccessible type
+
+**Error Code**
+
+INPUT_FIELD_REFERENCES_INACCESSIBLE_TYPE
+
+**Formal Specification**
+
+- Let {fields} be the set of all fields of the input types
+- For each {field} in {fields}:
+  - If {field} is not declared as `@inaccessible`
+    - Let {namedType} be the named type that {field} references
+    - {namedType} must not be declared as `@inaccessible`
+
+**Explanatory Text**
+
+In a composed schema, a field within an input type must only reference types
+that are exposed. This requirement guarantees that public types do not reference
+inaccessible structures which are intended for internal use.
+
+A valid case where a public input field references another public input type:
+
+```graphql example
+input Input1 {
+  field1: String!
+  field2: Input2
+}
+
+input Input2 {
+  field3: String
+}
+```
+
+Another valid case is where the field is not exposed in the composed schema:
+
+```graphql example
+input Input1 {
+  field1: String!
+  field2: Input2 @inaccessible
+}
+
+input Input2 @inaccessible {
+  field3: String
+}
+```
+
+An invalid case is when an input field references an inaccessible type:
+
+```graphql counter-example
+input Input1 {
+  field1: String!
+  field2: Input2!
+}
+
+input Input2 @inaccessible {
+  field3: String
+}
+```
+
 ## Validate Satisfiability