Try to use concepts

Let's see if this breaks things

CMakeLists.txt

@@ -52,7 +52,7 @@ option (TEQP_MULTICOMPLEX_ENABLED
 
 
 ####  SETUP
-set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD 20)
 
 set(ZIPFN "${CMAKE_CURRENT_SOURCE_DIR}/dev/docker/boost_bcp_docker/boost_teqp.tar.xz")
 set(OUTFN "${CMAKE_CURRENT_SOURCE_DIR}/boost_teqp/boost/version.hpp")

src/tests/catch_test_concepts.cxx

@@ -0,0 +1,44 @@
+#include <catch2/catch_test_macros.hpp>
+#include <catch2/catch_approx.hpp>
+#include <catch2/benchmark/catch_benchmark_all.hpp>
+using Catch::Approx;
+
+#include <iostream>
+#include <concepts>
+
+template<typename T>
+concept UsesTauDelta = requires(T t) {
+    { t.has_alphar_taudelta } -> std::same_as<std::true_type&>;
+};
+
+template<typename T>
+concept DoesntUseTauDelta = requires(T t) {
+    { t.has_alphar_taudelta } -> std::same_as<std::false_type&>;
+};
+
+void get(auto t){
+    if constexpr (UsesTauDelta<decltype(t)>){
+        std::cout << "Defined and used" << std::endl;
+    }
+    else if constexpr (DoesntUseTauDelta<decltype(t)>){
+        std::cout << "Defined and not used" << std::endl;
+    }
+    else{
+        std::cout << "unknown" << std::endl;
+    }
+}
+
+struct Adeftrue{
+    std::false_type has_alphar_taudelta;
+};
+
+struct Bdeffalse{
+    std::true_type has_alphar_taudelta;
+};
+struct C{
+};
+
+TEST_CASE("Test some C++20 things", "[C++20]"){
+    static_assert(DoesntUseTauDelta<Adeffalse>);
+    static_assert(UsesTauDelta<Adeftrue>);
+}