Adds Empty Object Type Validation Rule (#49)

Co-authored-by: PascalSenn <[email protected]>

spec/Section 4 -- Composition.md

@@ -18,4 +18,84 @@ run in sequence to produce the composite execution schema.
 
 ### Post Merge Validation
 
+#### Empty Merged Object Type
+
+**Error Code**
+
+`EMPTY_MERGED_OBJECT_TYPE`
+
+**Severity** ERROR
+
+**Formal Specification**
+
+- Let {types} be the set of all object types across all source schemas
+- For each {type} in {types}:
+  - {IsObjectTypeEmpty(type)} must be false.
+
+IsObjectTypeEmpty(type):
+
+- If {type} has `@inaccessible` directive
+- return false
+- Let {fields} be a set of all fields in {type}
+- For each {field} in {fields}:
+  - If {IsExposed(field)} is true
+    - return false
+- return true
+
+**Explanatory Text**
+
+For object types defined across multiple source schemas, the merged object type
+is the superset of all fields defined in these source schemas. However, any
+field marked with `@inaccessible` in any source schema is hidden and not
+included in the merged object type. An object type with no fields, after
+considering `@inaccessible` annotations, is considered empty and invalid.
+
+In the following example, the merged object type `ObjectType1` is valid. It
+includes all fields from both source schemas, with `field2` being hidden due to
+the `@inaccessible` directive in one of the source schemas:
+
+```graphql
+type ObjectType1 {
+  field1: String
+  field2: Int @inaccessible
+}
+
+type ObjectType1 {
+  field2: Int
+  field3: Boolean
+}
+```
+
+If the `@inaccessible` directive is applied to an object type itself, the entire
+merged object type is excluded from the composite execution schema, and it is
+not required to contain any fields.
+
+```graphql
+type ObjectType1 @inaccessible {
+  field1: String
+  field2: Int
+}
+
+type ObjectType1 {
+  field3: Boolean
+}
+```
+
+This counter-example demonstrates an invalid merged object type. In this case,
+`ObjectType1` is defined in two source schemas, but all fields are marked as
+`@inaccessible` in at least one of the source schemas, resulting in an empty
+merged object type:
+
+```graphql counter-example
+type ObjectType1 {
+  field1: String @inaccessible
+  field2: Boolean
+}
+
+type ObjectType1 {
+  field1: String
+  field2: Boolean @inaccessible
+}
+```
+
 ## Validate Satisfiability