Use a feature test macro to check for std::type_identity.

diff --git a/cmake/checks/check_01_cxx_features.cmake b/cmake/checks/check_01_cxx_features.cmake
index b6b9a0b28a0300f17f03d803dac250b2c6635b55..0bd45a0b1deeb81c885f4f74359af535f4b80c34 100644 (file)
--- a/cmake/checks/check_01_cxx_features.cmake
+++ b/cmake/checks/check_01_cxx_features.cmake
@@ -81,6 +81,10 @@ macro(_test_cxx20_support)
     #  error \"insufficient support for C++20\"
     #endif
 
+    #if !(defined __cpp_type_identity) || (__cpp_lib_type_identity < 201806)
+    #  error \"insufficient support for C++20\"
+    #endif
+
 
     // Test concepts and requires clauses
     template <int dim, int spacedim>