diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -537,7 +537,7 @@
   # in the dylib. C++20 is needed to use char8_t.
   set_target_properties(${target} PROPERTIES
     CXX_STANDARD 20
-    CXX_STANDARD_REQUIRED NO
+    CXX_STANDARD_REQUIRED YES
     CXX_EXTENSIONS NO)
 
   # When building the dylib, don't warn for unavailable aligned allocation
diff --git a/libcxx/benchmarks/CMakeLists.txt b/libcxx/benchmarks/CMakeLists.txt
--- a/libcxx/benchmarks/CMakeLists.txt
+++ b/libcxx/benchmarks/CMakeLists.txt
@@ -81,7 +81,7 @@
 set(BENCHMARK_NATIVE_INSTALL ${CMAKE_CURRENT_BINARY_DIR}/benchmark-native)
 
 add_library(               cxx-benchmarks-flags INTERFACE)
-target_compile_features(   cxx-benchmarks-flags INTERFACE cxx_std_17)
+target_compile_features(   cxx-benchmarks-flags INTERFACE cxx_std_20)
 target_compile_options(    cxx-benchmarks-flags INTERFACE -O2 -fsized-deallocation)
 target_include_directories(cxx-benchmarks-flags INTERFACE "${BENCHMARK_LIBCXX_INSTALL}/include"
                                                 INTERFACE "${LIBCXX_SOURCE_DIR}/test/support")