diff --git a/llvm/utils/gn/build/BUILD.gn b/llvm/utils/gn/build/BUILD.gn
--- a/llvm/utils/gn/build/BUILD.gn
+++ b/llvm/utils/gn/build/BUILD.gn
@@ -334,8 +334,9 @@
       "goma needs a sysroot: run `llvm/utils/sysroot.py make-fake --out-dir=sysroot` and add `sysroot = \"//sysroot\"` to your args.gn")
 
   if (use_ubsan) {
-    assert(is_clang && current_os == "linux",
-           "ubsan only supported on Linux/Clang")
+    assert(is_clang && (current_os == "ios" || current_os == "linux" ||
+                            current_os == "mac"),
+           "ubsan only supported on iOS/Clang, Linux/Clang, or macOS/Clang")
     cflags += [
       "-fsanitize=undefined",
       "-fno-sanitize-recover=all",
@@ -346,7 +347,7 @@
   if (use_asan) {
     assert(is_clang && (current_os == "ios" || current_os == "linux" ||
                             current_os == "mac"),
-           "asan only supported on iOS/Clang, Linux/Clang, and macOS/Clang")
+           "asan only supported on iOS/Clang, Linux/Clang, or macOS/Clang")
     cflags += [ "-fsanitize=address" ]
     ldflags += [ "-fsanitize=address" ]
   }
diff --git a/llvm/utils/gn/build/toolchain/BUILD.gn b/llvm/utils/gn/build/toolchain/BUILD.gn
--- a/llvm/utils/gn/build/toolchain/BUILD.gn
+++ b/llvm/utils/gn/build/toolchain/BUILD.gn
@@ -253,6 +253,12 @@
   toolchain_args = {
     current_os = "baremetal"
     current_cpu = "arm64"
+
+    # FIXME: These should be set in all toolchains building sanitizers,
+    # see discussion at https://reviews.llvm.org/D127906#3587329
+    use_asan = false
+    use_tsan = false
+    use_ubsan = false
   }
 }