Improve handling of Optional referenced types.

Author: brandonbloom

Sources/_OpenAPIGeneratorCore/Translator/CommonTranslations/translateTypealias.swift

@@ -28,7 +28,7 @@ extension FileTranslator {
         let typealiasDescription = TypealiasDescription(
             accessModifier: config.access,
             name: typeName.shortSwiftName,
-            existingType: .init(existingTypeUsage.withOptional(false))
+            existingType: .init(existingTypeUsage)
         )
         let typealiasComment: Comment? = typeName.docCommentWithUserDescription(userDescription)
         return .commentable(typealiasComment, .typealias(typealiasDescription))

Sources/_OpenAPIGeneratorCore/Translator/Parameters/translateParameter.swift

@@ -70,7 +70,7 @@ extension TypesFileTranslator {
         let decl = try translateSchema(
             typeName: typeName,
             schema: parameter.schema,
-            overrides: .init(isOptional: !parameter.required, userDescription: parameter.parameter.description)
+            overrides: .init(userDescription: parameter.parameter.description)
         )
         return decl
     }

Sources/_OpenAPIGeneratorCore/Translator/Responses/translateResponseHeader.swift

@@ -114,7 +114,7 @@ extension TypesFileTranslator {
         let decl = try translateSchema(
             typeName: typeName,
             schema: header.schema,
-            overrides: .init(isOptional: header.isOptional, userDescription: header.header.description)
+            overrides: .init(userDescription: header.header.description)
         )
         return decl
     }

Sources/_OpenAPIGeneratorCore/Translator/TypeAssignment/TypeAssigner.swift

@@ -345,7 +345,7 @@ struct TypeAssigner {
                 swiftComponent: asSwiftSafeName(originalName) + suffix,
                 jsonComponent: jsonReferenceComponentOverride ?? originalName
             )
-            .asUsage.withOptional(try typeMatcher.isOptional(schema, components: components))
+            .asUsage.withOptional(try typeMatcher.isOptionalRoot(schema, components: components))
     }
 
     /// Returns a type name for a reusable component.

Sources/_OpenAPIGeneratorCore/Translator/TypeAssignment/TypeMatcher.swift

@@ -78,7 +78,7 @@ struct TypeMatcher {
             },
             genericArrayHandler: { TypeName.arrayContainer.asUsage }
         )?
-        .withOptional(isOptional(schema, components: components))
+        .withOptional(isOptionalRoot(schema, components: components))
     }
 
     /// Returns a Boolean value that indicates whether the schema
@@ -331,6 +331,25 @@ struct TypeMatcher {
         return result
     }
 
+    /// Returns a Boolean value indicating whether the schema is optional at the root of any references.
+    /// - Parameters:
+    ///   - schema: The reference to check.
+    ///   - components: The OpenAPI components for looking up references.
+    /// - Throws: An error if there's an issue while checking the schema.
+    /// - Returns: `true` if the schema is an optional root, `false` otherwise.
+    func isOptionalRoot(_ schema: JSONSchema, components: OpenAPI.Components) throws -> Bool {
+        let directlyOptional = schema.nullable || !schema.required
+        switch schema.value {
+        case .null(_):
+            return true
+        case .reference(let ref, _):
+            let indirectlyOptional = try isOptional(ref, components: components)
+            return directlyOptional && !indirectlyOptional
+        default:
+            return directlyOptional
+        }
+    }
+
     // MARK: - Private
 
     /// Returns the type name of a built-in type that matches the specified

Tests/OpenAPIGeneratorReferenceTests/SnippetBasedReferenceTests.swift

@@ -237,7 +237,7 @@ final class SnippetBasedReferenceTests: XCTestCase {
             """,
             """
             public enum Schemas {
-                public typealias Null = OpenAPIRuntime.OpenAPIValueContainer
+                public typealias Null = OpenAPIRuntime.OpenAPIValueContainer?
                 public typealias NullArray = [Components.Schemas.Null]
             }
             """)
@@ -520,17 +520,17 @@ final class SnippetBasedReferenceTests: XCTestCase {
             """
             public enum Schemas {
                 public typealias MyRequiredString = Swift.String
-                public typealias MyNullableString = Swift.String
+                public typealias MyNullableString = Swift.String?
                 public struct MyObject: Codable, Hashable, Sendable {
                     public var id: Swift.Int64
                     public var alias: Swift.String?
                     public var requiredString: Components.Schemas.MyRequiredString
-                    public var nullableString: Components.Schemas.MyNullableString?
+                    public var nullableString: Components.Schemas.MyNullableString
                     public init(
                         id: Swift.Int64,
                         alias: Swift.String? = nil,
                         requiredString: Components.Schemas.MyRequiredString,
-                        nullableString: Components.Schemas.MyNullableString? = nil
+                        nullableString: Components.Schemas.MyNullableString
                     ) {
                         self.id = id
                         self.alias = alias