try clang 10 (#459)

* try clang 10

* use helics-builder clang 10 image

* try new image

* try again

* try different flag addition

* try adding cpp20 to visual studio check

* try with the latest flag enabled for visual studio 2019

* try the correct c++ flag

* remove use of std::result_of

* format files

azure-pipelines.yml

@@ -62,7 +62,8 @@ jobs:
         cli11.std: 11
       Windowslatest:
         vmImage: 'windows-2019'
-        cli11.std: 17
+        cli11.std: 20
+        cli11.options: -DCMAKE_CXX_FLAG="/std:c++latest"
   pool:
     vmImage: $(vmImage)
   steps:
@@ -113,6 +114,10 @@ jobs:
         containerImage: silkeh/clang:8
         cli11.std: 17
         cli11.options: -DCLI11_FORCE_LIBCXX=ON
+      clang10_20:
+        containerImage: helics/buildenv:clang10-builder
+        cli11.std: 20
+        cli11.options: -DCLI11_FORCE_LIBCXX=ON -DCMAKE_CXX_FLAGS=-std=c++20
   container: $[ variables['containerImage'] ]
   steps:
     - template: .ci/azure-cmake.yml

include/CLI/App.hpp

@@ -3055,7 +3055,18 @@ inline std::string help(const App *app, const Error &e) {
 namespace detail {
 /// This class is simply to allow tests access to App's protected functions
 struct AppFriend {
+#ifdef CLI11_CPP14
+
+    /// Wrap _parse_short, perfectly forward arguments and return
+    template <typename... Args> static decltype(auto) parse_arg(App *app, Args &&... args) {
+        return app->_parse_arg(std::forward<Args>(args)...);
+    }
 
+    /// Wrap _parse_subcommand, perfectly forward arguments and return
+    template <typename... Args> static decltype(auto) parse_subcommand(App *app, Args &&... args) {
+        return app->_parse_subcommand(std::forward<Args>(args)...);
+    }
+#else
     /// Wrap _parse_short, perfectly forward arguments and return
     template <typename... Args>
     static auto parse_arg(App *app, Args &&... args) ->
@@ -3069,6 +3080,7 @@ struct AppFriend {
         typename std::result_of<decltype (&App::_parse_subcommand)(App, Args...)>::type {
         return app->_parse_subcommand(std::forward<Args>(args)...);
     }
+#endif
     /// Wrap the fallthrough parent function to make sure that is working correctly
     static App *get_fallthrough_parent(App *app) { return app->_get_fallthrough_parent(); }
 };