run some tests and fixes for C++20 (#663)

Author: phlptp

azure-pipelines.yml

@@ -102,6 +102,9 @@ jobs:
       gcc9:
         containerImage: gcc:9
         cli11.std: 17
+      gcc11:
+        containerImage: gcc:11
+        cli11.std: 20
       gcc8:
         containerImage: gcc:8
         cli11.std: 17

include/CLI/App.hpp

@@ -799,9 +799,10 @@ class App {
 
     /// Add option for flag with integer result - defaults to allowing multiple passings, but can be forced to one
     /// if `multi_option_policy(CLI::MultiOptionPolicy::Throw)` is used.
-    template <typename T,
-              enable_if_t<std::is_constructible<T, std::int64_t>::value && !is_bool<T>::value, detail::enabler> =
-                  detail::dummy>
+    template <
+        typename T,
+        enable_if_t<std::is_constructible<T, std::int64_t>::value && !std::is_const<T>::value && !is_bool<T>::value,
+                    detail::enabler> = detail::dummy>
     Option *add_flag(std::string flag_name,
                      T &flag_count,  ///< A variable holding the count
                      std::string flag_description = "") {

tests/AppTest.cpp

@@ -2308,3 +2308,11 @@ TEST_CASE_METHOD(TApp, "logFormSingleDash", "[app]") {
     CHECK(veryverbose);
     CHECK(veryveryverbose);
 }
+
+TEST_CASE("C20_compile", "simple") {
+    CLI::App app{"test"};
+    auto flag = app.add_flag("--flag", "desc");
+
+    app.parse("--flag");
+    CHECK_FALSE(flag->empty());
+}

tests/OptionalTest.cpp

@@ -239,16 +239,19 @@ TEST_CASE_METHOD(TApp, "BoostOptionalEnumTest", "[optional]") {
     auto dstring = optptr->get_default_str();
     CHECK(dstring.empty());
     run();
-    CHECK(!opt);
+    auto checkOpt = static_cast<bool>(opt);
+    CHECK_FALSE(checkOpt);
 
     args = {"-v", "3"};
     run();
-    CHECK(opt);
+    checkOpt = static_cast<bool>(opt);
+    CHECK(checkOpt);
     CHECK(*opt == eval::val3);
     opt = {};
     args = {"--val", "1"};
     run();
-    CHECK(opt);
+    checkOpt = static_cast<bool>(opt);
+    CHECK(checkOpt);
     CHECK(*opt == eval::val1);
 }