From 3692fe63c183d50cf4468b8bf766ceded4674461 Mon Sep 17 00:00:00 2001
From: Randy Eckenrode <randy@largeandhighquality.com>
Date: Sun, 9 Aug 2026 08:38:12 -0400
Subject: [PATCH] llvmPackages_{18,19,20,21,22}.libllvm: backport Darwin triple
 parsing
MIME-Version: 1.0
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This fixes breakage on triples for macOS 27, which aligned the Darwin
version with the OS version (darwin26 is macOS 27, but so is darwin27,
which will be incorrectly interpreted as macOS 28 without this patch).
This is probably harmless in most cases, but it causes a test failure in
LLVM’s test suite when running under macOS 27.

(cherry picked from commit 6fb7a4dc0c9ba446ba0968bd046601a2e5903571)
---
 .../llvm/backport-darwin-triple-parsing.patch | 34 +++++++++++++++++++
 .../llvm/backport-darwin-triple-parsing.patch | 31 +++++++++++++++++
 .../compilers/llvm/common/llvm/default.nix    |  9 ++++-
 .../compilers/llvm/common/patches.nix         | 11 ++++++
 4 files changed, 84 insertions(+), 1 deletion(-)
 create mode 100644 pkgs/development/compilers/llvm/18/llvm/backport-darwin-triple-parsing.patch
 create mode 100644 pkgs/development/compilers/llvm/21/llvm/backport-darwin-triple-parsing.patch

diff --git a/pkgs/development/compilers/llvm/18/llvm/backport-darwin-triple-parsing.patch b/pkgs/development/compilers/llvm/18/llvm/backport-darwin-triple-parsing.patch
new file mode 100644
index 000000000000..9426f99512b7
--- /dev/null
+++ b/pkgs/development/compilers/llvm/18/llvm/backport-darwin-triple-parsing.patch
@@ -0,0 +1,34 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Randy Eckenrode <randy@largeandhighquality.com>
+Date: Tue, 11 Aug 2026 20:12:42 -0400
+Subject: [PATCH] backport-darwin-triple-parsing
+
+---
+ lib/TargetParser/Triple.cpp | 10 ++++++++--
+ 1 file changed, 8 insertions(+), 2 deletions(-)
+
+diff --git a/lib/TargetParser/Triple.cpp b/lib/TargetParser/Triple.cpp
+index 0bbe8a3cedfd..4143c3eac05d 100644
+--- a/lib/TargetParser/Triple.cpp
++++ b/lib/TargetParser/Triple.cpp
+@@ -1253,9 +1253,15 @@ bool Triple::getMacOSXVersion(VersionTuple &Version) const {
+     }
+     if (Version.getMajor() <= 19) {
+       Version = VersionTuple(10, Version.getMajor() - 4);
+-    } else {
+-      // darwin20+ corresponds to macOS 11+.
++    } else if (Version.getMajor() < 25) {
++      // darwin20-24 corresponds to macOS 11-15.
+       Version = VersionTuple(11 + Version.getMajor() - 20);
++    } else if ((Version.getMajor() == 25) || (Version.getMajor() == 26)) {
++      // darwin25-26 corresponds to macOS 26-27.
++      Version = VersionTuple(Version.getMajor() + 1);
++    } else {
++      // Starting with darwin27, it naturally corresponds to the same macOS
++      // version.
+     }
+     break;
+   case MacOSX:
+-- 
+2.54.0
+
diff --git a/pkgs/development/compilers/llvm/21/llvm/backport-darwin-triple-parsing.patch b/pkgs/development/compilers/llvm/21/llvm/backport-darwin-triple-parsing.patch
new file mode 100644
index 000000000000..7f2cc06b51d4
--- /dev/null
+++ b/pkgs/development/compilers/llvm/21/llvm/backport-darwin-triple-parsing.patch
@@ -0,0 +1,31 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Randy Eckenrode <randy@largeandhighquality.com>
+Date: Tue, 11 Aug 2026 20:19:29 -0400
+Subject: [PATCH] backport-darwin-triple-parsing
+
+---
+ lib/TargetParser/Triple.cpp | 7 +++++--
+ 1 file changed, 5 insertions(+), 2 deletions(-)
+
+diff --git a/lib/TargetParser/Triple.cpp b/lib/TargetParser/Triple.cpp
+index 0584c941d2e6..30cf78dbaabc 100644
+--- a/lib/TargetParser/Triple.cpp
++++ b/lib/TargetParser/Triple.cpp
+@@ -1449,9 +1449,12 @@ bool Triple::getMacOSXVersion(VersionTuple &Version) const {
+     } else if (Version.getMajor() < 25) {
+       // darwin20-24 corresponds to macOS 11-15.
+       Version = VersionTuple(11 + Version.getMajor() - 20);
+-    } else {
+-      // darwin25 corresponds with macOS26+.
++    } else if ((Version.getMajor() == 25) || (Version.getMajor() == 26)) {
++      // darwin25-26 corresponds to macOS 26-27.
+       Version = VersionTuple(Version.getMajor() + 1);
++    } else {
++      // Starting with darwin27, it naturally corresponds to the same macOS
++      // version.
+     }
+     break;
+   case MacOSX:
+-- 
+2.54.0
+
diff --git a/pkgs/development/compilers/llvm/common/llvm/default.nix b/pkgs/development/compilers/llvm/common/llvm/default.nix
index d5dcd4d58cce..222f7fb6e54d 100644
--- a/pkgs/development/compilers/llvm/common/llvm/default.nix
+++ b/pkgs/development/compilers/llvm/common/llvm/default.nix
@@ -222,7 +222,14 @@ stdenv.mkDerivation (
               hash = "sha256-3hkbYPUVRAtWpo5qBmc2jLZLivURMx8T0GQomvNZesc=";
               stripLen = 1;
             }
-          );
+          )
+      ++ lib.optionals (lib.versionOlder release_version "23") [
+        # As of macOS 27 (and iOS 27, etc), the Darwin version number is the same as the OS version number.
+        # This change breaks target parsing because `darwin27` is incorrectly interpreted as macOS 28.
+        # This patch is a backport of the target parsing changes in LLVM 23, which fixes the problem.
+        # Hopefully, Apple does not change the version number scheme again any time soon.
+        (getVersionFile "llvm/backport-darwin-triple-parsing.patch")
+      ];
 
     nativeBuildInputs = [
       cmake
diff --git a/pkgs/development/compilers/llvm/common/patches.nix b/pkgs/development/compilers/llvm/common/patches.nix
index 76ae90c3aaa7..5ea9faa3d2e2 100644
--- a/pkgs/development/compilers/llvm/common/patches.nix
+++ b/pkgs/development/compilers/llvm/common/patches.nix
@@ -20,6 +20,17 @@
       path = ../18;
     }
   ];
+  "llvm/backport-darwin-triple-parsing.patch" = [
+    {
+      after = "18";
+      before = "21";
+      path = ../18;
+    }
+    {
+      after = "21";
+      path = ../21;
+    }
+  ];
   "llvm/gnu-install-dirs.patch" = [
     {
       after = "23";