Nullable enums with an empty string fail to get generated (#176)

Nullable enums with an empty string fail to get generated

### Motivation

Fixes #118.

The issue is that when a `type: string, enum: ...` schema is marked as `nullable: true`, _empty cases are parsed as Void_ instead of _nil_. See the issue for the investigation and identifying the root cause, but the TL;DR is that we need a workaround in the generator in the short term, and it's uncertain if the issue will ever be addressed in our dependency Yams (might be unlikely, as it'd probably be a breaking change for them at this point).

### Modifications

This PR adds code that catches this case and converts the `Void` into an empty string, which is what the OpenAPI document author actually put into their document in the first place.

### Result

Now `nullable: true` string enums with an empty case can be generated correctly. One example of such use is the [GitHub API](https://github.com/github/rest-api-description).

Note that this PR isn't really adding support for nullable schemas in general, that's tracked by #82. We're just working around one specific scenario in which a relatively common input trips up the generator.

### Test Plan

Added a unit test for each of: non-nullable and nullable variants.


Reviewed by: gjcairo, simonjbeaumont

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#176

Sources/_OpenAPIGeneratorCore/Translator/CommonTranslations/translateSchema.swift

@@ -115,6 +115,7 @@ extension FileTranslator {
             let enumDecl = try translateStringEnum(
                 typeName: typeName,
                 userDescription: overrides.userDescription ?? coreContext.description,
+                isNullable: coreContext.nullable,
                 allowedValues: allowedValues
             )
             return [enumDecl]

Sources/_OpenAPIGeneratorCore/Translator/CommonTranslations/translateStringEnum.swift

@@ -20,15 +20,23 @@ extension FileTranslator {
     ///   - typeName: The name of the type to give to the declared enum.
     ///   - openAPIDescription: A user-specified description from the OpenAPI
     ///   document.
+    ///   - isNullable: Whether the enum schema is nullable.
     ///   - allowedValues: The enumerated allowed values.
     func translateStringEnum(
         typeName: TypeName,
         userDescription: String?,
+        isNullable: Bool,
         allowedValues: [AnyCodable]
     ) throws -> Declaration {
 
         let rawValues = try allowedValues.map(\.value)
             .map { anyValue in
+                // In nullable enum schemas, empty strings are parsed as Void.
+                // This is unlikely to be fixed, so handling that case here.
+                // https://github.com/apple/swift-openapi-generator/issues/118
+                if isNullable && anyValue is Void {
+                    return ""
+                }
                 guard let string = anyValue as? String else {
                     throw GenericError(message: "Disallowed value for a string enum '\(typeName)': \(anyValue)")
                 }

Tests/OpenAPIGeneratorCoreTests/Translator/CommonTranslations/Test_translateStringEnum.swift

@@ -0,0 +1,63 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftOpenAPIGenerator open source project
+//
+// Copyright (c) 2023 Apple Inc. and the SwiftOpenAPIGenerator project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftOpenAPIGenerator project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+import XCTest
+import OpenAPIKit30
+@testable import _OpenAPIGeneratorCore
+
+final class Test_translateStringEnum: Test_Core {
+
+    func testCaseValues() throws {
+        let names = try _caseValues(
+            .string(
+                allowedValues: "a",
+                ""
+            )
+        )
+        XCTAssertEqual(names, ["a", "_empty", "undocumented"])
+    }
+
+    func testCaseValuesForNullableSchema() throws {
+        let names = try _caseValues(
+            .string(
+                nullable: true,
+                allowedValues: "a",
+                nil
+            )
+        )
+        XCTAssertEqual(names, ["a", "_empty", "undocumented"])
+    }
+
+    func _caseValues(_ schema: JSONSchema) throws -> [String] {
+        self.continueAfterFailure = false
+        let translator = makeTypesTranslator()
+        let decls = try translator.translateSchema(
+            typeName: .init(swiftKeyPath: ["FooEnum"]),
+            schema: schema,
+            overrides: .none
+        )
+        XCTAssertEqual(decls.count, 1)
+        let decl = decls[0]
+        guard case .enum(let enumDesc) = decl.strippingTopComment else {
+            throw UnexpectedDeclError(actual: decl.info.kind, expected: .enum)
+        }
+        XCTAssertEqual(enumDesc.name, "FooEnum")
+        let names: [String] = enumDesc.members.compactMap { memberDecl in
+            guard case .enumCase(let caseDesc) = memberDecl.strippingTopComment else {
+                return nil
+            }
+            return caseDesc.name
+        }
+        return names
+    }
+}