diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index edb6087..45f0c0f 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -152,7 +152,14 @@ subprocess_set_cxx_flags(test98.cpp "-std=gnu++98")
 subprocess_set_cxx_flags(test11.cpp "-std=c++11" "-Wno-c++98-compat")
 subprocess_set_cxx_flags(test14.cpp "-std=c++14" "-Wno-c++98-compat")
 subprocess_set_cxx_flags(test17.cpp "-std=c++17" "-Wno-c++98-compat")
-subprocess_set_cxx_flags(test20.cpp "-std=c++20" "-Wno-c++98-compat")
+# GCC 8 and clang 9 only accept the pre-release spelling -std=c++2a.
+include(CheckCXXCompilerFlag)
+check_cxx_compiler_flag("-std=c++20" SUBPROCESS_CXX_HAS_STD_CXX20)
+if(SUBPROCESS_CXX_HAS_STD_CXX20)
+  subprocess_set_cxx_flags(test20.cpp "-std=c++20" "-Wno-c++98-compat")
+else()
+  subprocess_set_cxx_flags(test20.cpp "-std=c++2a" "-Wno-c++98-compat")
+endif()
 subprocess_set_cxx_flags(pre_windows.cpp  "" "-Wno-c++98-compat")
 subprocess_set_cxx_flags(post_windows.cpp "" "-Wno-c++98-compat")