From 8ed9bb36679a342e542bd68eab14ec54d52351d9 Mon Sep 17 00:00:00 2001
From: John Szumski <jszumski@squareup.com>
Date: Fri, 2 May 2025 13:16:29 -0400
Subject: [PATCH] Add a way to map a workflow's Never output type to a
 different type without generating compiler warnings

---
 Workflow/Sources/AnyWorkflow.swift    | 11 +++++++++++
 Workflow/Tests/AnyWorkflowTests.swift | 17 +++++++++++++++++
 2 files changed, 28 insertions(+)

diff --git a/Workflow/Sources/AnyWorkflow.swift b/Workflow/Sources/AnyWorkflow.swift
index 0a76e7500..852d318ac 100644
--- a/Workflow/Sources/AnyWorkflow.swift
+++ b/Workflow/Sources/AnyWorkflow.swift
@@ -195,3 +195,14 @@ extension AnyWorkflowConvertible {
         asAnyWorkflow().mapRendering(transform)
     }
 }
+
+// MARK: -
+
+extension AnyWorkflowConvertible where Output == Never {
+    /// A convenience for workflows that don't produce output but want to change the declared output type.
+    public func mapOutput<NewOutput>(to: NewOutput.Type) -> AnyWorkflow<Rendering, NewOutput> {
+        mapOutput(mapNever(_:))
+    }
+}
+
+private func mapNever<T>(_ input: Never) -> T {}
diff --git a/Workflow/Tests/AnyWorkflowTests.swift b/Workflow/Tests/AnyWorkflowTests.swift
index 4dee01eb2..fdd448375 100644
--- a/Workflow/Tests/AnyWorkflowTests.swift
+++ b/Workflow/Tests/AnyWorkflowTests.swift
@@ -82,6 +82,13 @@ public class AnyWorkflowTests: XCTestCase {
 
         XCTAssertNotNil(erased.base as? OnOutputWorkflow)
     }
+
+    func testMapNeverToOtherType() {
+        // test that this `Output = Bool` workflow compiles when mapped from an `Output = Never` workflow
+        let boolOutputWorkflow: AnyWorkflow<String, Bool> = NeverOutputWorkflow().mapOutput(to: Bool.self)
+
+        XCTAssertNotNil(boolOutputWorkflow)
+    }
 }
 
 /// Has no state or output, simply renders a reversed string
@@ -150,3 +157,13 @@ private struct OnOutputChildWorkflow: Workflow {
         }
     }
 }
+
+private struct NeverOutputWorkflow: Workflow {
+    typealias Output = Never
+    typealias Rendering = String
+    typealias State = Void
+
+    func render(state: State, context: RenderContext<NeverOutputWorkflow>) -> String {
+        "Never"
+    }
+}