Support {oneOf: [{type: 'null'}, ...]} as optional

Fixes apple#513

Sources/_OpenAPIGeneratorCore/Translator/CommonTranslations/translateAllAnyOneOf.swift

@@ -38,7 +38,7 @@ extension TypesFileTranslator {
     /// - Throws: An error if there is an issue during translation.
     /// - Returns: A declaration representing the translated allOf/anyOf structure.
     func translateAllOrAnyOf(typeName: TypeName, openAPIDescription: String?, type: AllOrAnyOf, schemas: [JSONSchema])
-        throws -> Declaration
+        throws -> [Declaration]
     {
         let properties: [(property: PropertyBlueprint, isKeyValuePair: Bool)] = try schemas.enumerated()
             .map { index, schema in
@@ -107,7 +107,7 @@ extension TypesFileTranslator {
                 properties: propertyValues
             )
         )
-        return structDecl
+        return [structDecl]
     }
 
     /// Returns a declaration for a oneOf schema.
@@ -128,7 +128,7 @@ extension TypesFileTranslator {
         openAPIDescription: String?,
         discriminator: OpenAPI.Discriminator?,
         schemas: [JSONSchema]
-    ) throws -> Declaration {
+    ) throws -> [Declaration] {
         let cases: [(String, [String]?, Bool, Comment?, TypeUsage, [Declaration])]
         if let discriminator {
             // > When using the discriminator, inline schemas will not be considered.
@@ -148,7 +148,15 @@ extension TypesFileTranslator {
                 return (caseName, mappedType.rawNames, true, comment, mappedType.typeName.asUsage, [])
             }
         } else {
-            cases = try schemas.enumerated()
+            let (schemas, nullSchemas) = schemas.partitioned(by: { typeMatcher.isNull($0) })
+            if schemas.count == 1, nullSchemas.count > 0, let schema = schemas.first {
+                return try translateSchema(
+                    typeName: typeName,
+                    schema: schema,
+                    overrides: .init(isOptional: true))
+            }
+            cases = try schemas
+                .enumerated()
                 .map { index, schema in
                     let key = "case\(index+1)"
                     let childType = try typeAssigner.typeUsage(
@@ -242,6 +250,6 @@ extension TypesFileTranslator {
             conformances: Constants.ObjectStruct.conformances,
             members: caseDecls + codingKeysDecls + [decoder, encoder]
         )
-        return .commentable(comment, enumDecl)
+        return [.commentable(comment, enumDecl)]
     }
 }

Sources/_OpenAPIGeneratorCore/Translator/CommonTranslations/translateSchema.swift

@@ -141,29 +141,26 @@ extension TypesFileTranslator {
                 arrayContext: arrayContext
             )
         case let .all(of: schemas, core: coreContext):
-            let allOfDecl = try translateAllOrAnyOf(
+            return try translateAllOrAnyOf(
                 typeName: typeName,
                 openAPIDescription: overrides.userDescription ?? coreContext.description,
                 type: .allOf,
                 schemas: schemas
             )
-            return [allOfDecl]
         case let .any(of: schemas, core: coreContext):
-            let anyOfDecl = try translateAllOrAnyOf(
+            return try translateAllOrAnyOf(
                 typeName: typeName,
                 openAPIDescription: overrides.userDescription ?? coreContext.description,
                 type: .anyOf,
                 schemas: schemas
             )
-            return [anyOfDecl]
         case let .one(of: schemas, core: coreContext):
-            let oneOfDecl = try translateOneOf(
+            return try translateOneOf(
                 typeName: typeName,
                 openAPIDescription: overrides.userDescription ?? coreContext.description,
                 discriminator: coreContext.discriminator,
                 schemas: schemas
             )
-            return [oneOfDecl]
         default: return []
         }
     }

Sources/_OpenAPIGeneratorCore/Translator/TypeAssignment/TypeMatcher.swift

@@ -350,6 +350,21 @@ struct TypeMatcher {
         }
     }
 
+    /// Returns a Boolean value indicating whether the schema admits only explicit null values.
+    /// - Parameters:
+    ///   - schema: The schema to check.
+    /// - Returns: `true` if the schema admits only explicit null values, `false` otherwise.
+    func isNull(_ schema: JSONSchema) -> Bool {
+        switch schema.value {
+        case .null(_):
+            return true
+        case let .fragment(core):
+            return core.format.jsonType == .null
+        default:
+            return false
+        }
+    }
+
     // MARK: - Private
 
     /// Returns the type name of a built-in type that matches the specified

Tests/OpenAPIGeneratorReferenceTests/SnippetBasedReferenceTests.swift

@@ -1761,6 +1761,31 @@ final class SnippetBasedReferenceTests: XCTestCase {
         )
     }
 
+    func testOneOfRefOrNull() throws {
+        try self.assertSchemasTranslation(
+            """
+            schemas:
+              SomeString:
+                type: string
+              NullableRef:
+                oneOf:
+                  - $ref: '#/components/schemas/SomeString'
+                  - type: 'null'
+              ArrayOfNullableRefs:
+                type: array
+                items:
+                  $ref: '#/components/schemas/NullableRef'
+            """,
+            """
+            public enum Schemas {
+                public typealias SomeString = Swift.String
+                public typealias NullableRef = Components.Schemas.SomeString?
+                public typealias ArrayOfNullableRefs = [Components.Schemas.NullableRef]
+            }
+            """
+        )
+    }
+
     func testComponentsResponsesResponseNoBody() throws {
         try self.assertResponsesTranslation(
             """