add trait.hpp.

mlib.vcxproj

@@ -160,6 +160,7 @@
     <ClInclude Include="select.hpp" />
     <ClInclude Include="string_manip.hpp" />
     <ClInclude Include="times.hpp" />
+    <ClInclude Include="trait.hpp" />
     <ClInclude Include="transform.hpp" />
     <ClInclude Include="tuple.hpp" />
     <ClInclude Include="tuple_inheritance.hpp" />

mlib.vcxproj.filters

@@ -123,6 +123,9 @@
     <ClInclude Include="times.hpp">
       <Filter>Header Files</Filter>
     </ClInclude>
+    <ClInclude Include="trait.hpp">
+      <Filter>Header Files</Filter>
+    </ClInclude>
   </ItemGroup>
   <ItemGroup>
     <ClCompile Include="Main.cpp">

trait.hpp

@@ -0,0 +1,26 @@
+#pragma once
+// only works on Clang and MSVC, not GCC
+
+namespace mlib
+{
+    template<auto T>
+    struct trait_module
+    {
+        constexpr auto operator()(auto X) const
+        {
+            // we call our NTTP
+            T(X);
+            return true;
+        }
+    };
+
+
+    template<auto T, auto... trait_modules>
+        requires((trait_modules(T), ...))
+    struct trait
+    {
+        constexpr trait() {}
+
+        static constexpr auto value = true;
+    };
+} // namespace mlib