Adds Enum Type Default Value Uses Inaccessible Value Rule

spec/Section 4 -- Composition.md

@@ -14,6 +14,111 @@ run in sequence to produce the composite execution schema.
 
 ### Pre Merge Validation
 
+#### Enum Type Default Value Uses Inaccessible Value
+
+**Error Code**
+
+`ENUM_TYPE_DEFAULT_VALUE_INACCESSIBLE`
+
+**Formal Specification**
+
+- {ValidateArgumentDefaultValues()} must be true.
+- {ValidateInputFieldDefaultValues()} must be true.
+
+ValidateArgumentDefaultValues():
+
+- Let {arguments} be all arguments of fields and directives across all subgraphs
+- For each {argument} in {arguments}
+  - If {IsExposed(argument)} is true and has a default value:
+    - Let {defaultValue} be the default value of {argument}
+    - If not ValidateDefaultValue(defaultValue)
+      - return false
+- return true
+
+ValidateInputFieldDefaultValues():
+
+- Let {inputFields} be all input fields of across all subgraphs
+- For each {inputField} in {inputFields}:
+  - Let {type} be the type of {inputField}
+  - If {IsExposed(inputField)} is true and {inputField} has a default value:
+    - Let {defaultValue} be the default value of {inputField}
+    - if ValidateDefaultValue(defaultValue) is false
+      - return false
+- return true
+
+ValidateDefaultValue(defaultValue):
+
+- If {defaultValue} is a ListValue:
+  - For each {valueNode} in {defaultValue}:
+    - If {ValidateDefaultValue(valueNode)} is false
+      - return false
+- If {defaultValue} is an ObjectValue:
+  - Let {objectFields} be a list of all fields of {defaultValue}
+  - Let {fields} be a list of all fields {objectFields} are referring to
+  - For each {field} in {fields}:
+    - If {IsExposed(field)} is false
+      - return false
+  - For each {objectField} in {objectFields}:
+    - Let {value} be the value of {objectField}
+    - return {ValidateDefaultValue(value)}
+- If {defaultValue} is an EnumValue:
+  - If {IsExposed(defaultValue)} is false
+    - return false
+- return true
+
+**Explanatory Text**
+
+This rule ensures that inaccessible enum values are not exposed in the composed schema through default values.
+Output field arguments, input fields and directive arguments must only use enum values as their default value that is not annotated with the `@inaccessible` directive.
+
+In this example the `FOO` value in the `Enum1` enum is not marked with
+`@inaccessible`, hence it doesn't violate the rule.
+
+```graphql
+type Query {
+  field(type: Enum1 = FOO): [Baz!]!
+}
+
+enum Enum1 {
+  FOO
+  BAR
+}
+```
+
+This following example violates this rule because the default value for the field
+`field` in type `Input1` references an enum value (`FOO`) that is marked as
+`@inaccessible`.
+
+```graphql counter-example
+type Query {
+  field(arg: Enum1 = FOO): [Baz!]!
+}
+
+input Input1 {
+  field: Enum1 = FOO
+}
+
+directive @directive1(arg: Enum1 = FOO) on FIELD_DEFINITION
+
+enum Enum1 {
+  FOO @inaccessible
+  BAR
+}
+```
+
+```graphql counter-example
+type Query {
+  field(arg: Input1 = { field2: "ERROR" }): [Baz!]!
+}
+
+directive @directive1(arg: Input1 = { field2: "ERROR" }) on FIELD_DEFINITION
+
+input Input1 {
+  field1: String
+  field2: String @inaccessible
+}
+```
+
 ### Merge
 
 ### Post Merge Validation