Adds empty merged input object type to composition

spec/Section 4 -- Composition.md

@@ -18,4 +18,49 @@ run in sequence to produce the composite execution schema.
 
 ### Post Merge Validation
 
+#### Empty Merged Input Object Type
+
+**Error Code**
+
+INPUT_OBJECT_TYPE_EMPTY
+
+**Formal Specification**
+
+- Let {inputs} be the set of all input object in the merged schema
+- For each {input} in {inputs}:
+  - {IsInputObjectTypeEmpty(input)} must be false
+
+IsInputObjectTypeEmpty(input):
+
+- Let {fields} be a set of all input fields of {input}
+- For each {field} in {fields}:
+  - If {field} is not flagged as `@inaccessible`:
+    - return false
+- return true
+
+**Explanatory Text**
+
+When an input object type is defined in multiple source schemas, fields flagged as `@inaccessible` are not included in the merged input object type. 
+If this process results in an input object type with no fields, the input object type is considered empty and invalid.
+
+In the following example, the merged input object type `Input1` is valid. 
+
+```graphql
+input Input1 {
+  field1: String @inaccessible
+  field2: Int
+}
+```
+
+This counter-example shows an invalid merged input object type. 
+The `Input1` type is defined in two source schemas, but both `field1` and `field2` are flagged as
+`@inaccessible` in at least on of the schemas:
+
+```graphql counter-example
+input Input1 {
+  field1: String @inaccessible
+  field2: Int @inaccessible
+}
+```
+
 ## Validate Satisfiability