Fix a C++20 cmake check.

diff --git a/cmake/checks/check_01_cxx_features.cmake b/cmake/checks/check_01_cxx_features.cmake
index 49a44ccceeadcd1d8549853a03e1c14b3e5e2ecd..4dc09a585040a972b8fbbf95bd438beff8b6f8d9 100644 (file)
--- a/cmake/checks/check_01_cxx_features.cmake
+++ b/cmake/checks/check_01_cxx_features.cmake
@@ -81,10 +81,13 @@ 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\"
+    #if !(defined __cpp_lib_type_identity)
+    #  error \"insufficient support for C++20: __cpp_lib_type_identity not defined\"
     #endif
 
+    #if !(defined __cpp_lib_type_identity) || (__cpp_lib_type_identity < 201806)
+    #  error \"insufficient support for C++20: __cpp_lib_type_identity is too old \"
+    #endif
 
     // Test concepts and requires clauses
     template <int dim, int spacedim>