From 0a409130d3f007de9caaaa9fc423e800a41bec9d Mon Sep 17 00:00:00 2001
From: Evan Wilde <ewilde@apple.com>
Date: Fri, 19 Sep 2025 16:51:06 -0700
Subject: [PATCH] Fix Bootstrap: diffing

The pass that tracks which control-flow positions are unreachable after
infinite loops was migrated to pure Swift in PR #79186. As a result, the
C++-only compiler used to bootstrap is unable to determine that the
code-location after loops is unreachable. Placing a fatalError after the
infinite while loop.

Fixes: rdar://160956325
---
 stdlib/public/core/Diffing.swift | 1 +
 1 file changed, 1 insertion(+)

diff --git a/stdlib/public/core/Diffing.swift b/stdlib/public/core/Diffing.swift
index a6991907d4638..d15a63f2e15af 100644
--- a/stdlib/public/core/Diffing.swift
+++ b/stdlib/public/core/Diffing.swift
@@ -388,6 +388,7 @@ fileprivate struct LinearMyers: ~Copyable {
       // Save the right side of the snake for later
       stack.append(tailBox)
     }
+    fatalError("Unreachable")
   }
   
   fileprivate mutating func middleSnake<T>(