diff --git a/share/mk/src.opts.mk b/share/mk/src.opts.mk
--- a/share/mk/src.opts.mk
+++ b/share/mk/src.opts.mk
@@ -348,6 +348,11 @@
 __DEFAULT_NO_OPTIONS+=OPENMP
 .endif
 
+# libc++ requires C++20
+.if !${COMPILER_FEATURES:Mc++20}
+BROKEN_OPTIONS+=CXX
+.endif
+
 .include <bsd.mkopt.mk>
 
 #