Adds Output Field Types Mergable to Composition

spec/Section 4 -- Composition.md

@@ -14,6 +14,107 @@ run in sequence to produce the composite execution schema.
 
 ### Pre Merge Validation
 
+#### Output Field Types Mergeable
+
+**Error Code**
+
+OUTPUT_FIELD_TYPES_NOT_MERGEABLE
+
+**Severity**
+
+ERROR
+
+**Formal Specification**
+
+- Let {typeNames} be the set of all output type names from all source schemas.
+- For each {typeName} in {typeNames}
+  - Let {types} be the set of all types with the name {typeName} from all source
+    schemas.
+  - Let {fieldNames} be the set of all field names from all {types}.
+  - For each {fieldName} in {fieldNames}
+    - Let {fields} be the set of all fields with the name {fieldName} from all
+      {types}.
+    - {FieldsAreMergeable(fields)} must be true.
+
+FieldsAreMergeable(fields):
+
+- Given each pair of members {fieldA} and {fieldB} in {fields}:
+  - Let {typeA} be the type of {fieldA}
+  - Let {typeB} be the type of {fieldB}
+  - {SameOutputTypeShape(typeA, typeB)} must be true.
+
+**Explanatory Text**
+
+Fields on objects or interfaces that have the same name are considered
+semantically equivalent and mergeable when they have a mergeable field type.
+
+Fields with the same type are mergeable.
+
+```graphql example
+type User {
+  birthdate: String
+}
+
+type User {
+  birthdate: String
+}
+```
+
+Fields with different nullability are mergeable, resulting in a merged field
+with a nullable type.
+
+```graphql example
+type User {
+  birthdate: String!
+}
+
+type User {
+  birthdate: String
+}
+```
+
+```graphql example
+type User {
+  tags: [String!]
+}
+
+type User {
+  tags: [String]!
+}
+
+type User {
+  tags: [String]
+}
+```
+
+Fields are not mergeable if the named types are different in kind or name.
+
+```graphql counter-example
+type User {
+  birthdate: String!
+}
+
+type User {
+  birthdate: DateTime!
+}
+```
+
+```graphql counter-example
+type User {
+  tags: [Tag]
+}
+
+type Tag {
+  value: String
+}
+
+type User {
+  tags: [Tag]
+}
+
+scalar Tag
+```
+
 ### Merge
 
 ### Post Merge Validation