diff --git a/toolchains/BUILD.gn b/toolchains/BUILD.gn
index 6f74757..8287c00 100644
--- a/toolchains/BUILD.gn
+++ b/toolchains/BUILD.gn
@@ -31,6 +31,9 @@
     "-std=c99",
     "-pedantic",
   ]
+
+  # Modern C++ for test and tooling.
+  cflags_cc = [ "-std=c++20" ]
 }
 
 config("enable_sanitizers") {