From 7ca59d30fde32f7b815dee92fef1fbe16d5336b8 Mon Sep 17 00:00:00 2001
From: Matt Jolly <kangie@gentoo.org>
Date: Sun, 19 Apr 2026 08:42:04 +1000
Subject: [PATCH] build: gate -fsanitize-ignore-for-ubsan-feature behind
 clang_has_ubsan_feature_ignore

The -fsanitize-ignore-for-ubsan-feature flag was added in LLVM 23. Older
toolchains reject it as an unknown argument, breaking the build when the
ubsan_hardening() template is used.

Add a clang_has_ubsan_feature_ignore GN arg (default true) that gates
emission of the flag. Set it to false for toolchains predating LLVM 23.

The -fsanitize= and -fsanitize-trap= flags remain unconditional, so the
actual hardening still applies on older compilers.

Signed-off-by: Matt Jolly <kangie@gentoo.org>
--- a/build/config/sanitizers/sanitizers.gni
+++ b/build/config/sanitizers/sanitizers.gni
@@ -495,6 +495,12 @@ if ((is_apple || is_win) && (is_asan || is_ubsan_any)) {
   clang_rt_dso = get_path_info(clang_rt_dso_path, "file")
 }
 
+declare_args() {
+  # Set to false when using a Clang that predates
+  # -fsanitize-ignore-for-ubsan-feature (added in LLVM 23).
+  clang_has_ubsan_feature_ignore = true
+}
+
 # Adds hardening by way of UBSan instrumentation. No handling is
 # provided: your target will crash when the undefined behavior is
 # detected.
@@ -534,14 +540,17 @@ template("ubsan_hardening") {
       cflags = [
         "-fsanitize=${invoker.sanitizer}",
         "-fsanitize-trap=${invoker.sanitizer}",
-
+      ]
+      if (clang_has_ubsan_feature_ignore) {
         # Prevents `__has_feature(undefined_behavior_sanitizer)`
         # from evaluating true. Configs defined here are intended to
         # be usable even in release builds, i.e. as widely as possible.
         # It's important not to have full-on UBSan workarounds activate
         # just because we built support for a specific sanitizer.
-        "-fsanitize-ignore-for-ubsan-feature=${invoker.sanitizer}",
-      ]
+        cflags += [
+          "-fsanitize-ignore-for-ubsan-feature=${invoker.sanitizer}",
+        ]
+      }
       if (defined(invoker.cflags)) {
         cflags += invoker.cflags
       }
-- 
2.53.0