String conversions (#421)

* add a variant of the string conversions for the default string operations.  Discriminate between the is_convertible and is_constructible type traits for object.

* update the test to test the different situations with the funny string like type

include/CLI/TypeTools.hpp

@@ -219,15 +219,24 @@ template <typename S> class is_tuple_like {
 };
 
 /// Convert an object to a string (directly forward if this can become a string)
-template <typename T, enable_if_t<std::is_constructible<std::string, T>::value, detail::enabler> = detail::dummy>
+template <typename T, enable_if_t<std::is_convertible<T, std::string>::value, detail::enabler> = detail::dummy>
 auto to_string(T &&value) -> decltype(std::forward<T>(value)) {
     return std::forward<T>(value);
 }
 
+/// Construct a string from the object
+template <typename T,
+          enable_if_t<std::is_constructible<std::string, T>::value && !std::is_convertible<T, std::string>::value,
+                      detail::enabler> = detail::dummy>
+std::string to_string(const T &value) {
+    return std::string(value);
+}
+
 /// Convert an object to a string (streaming must be supported for that type)
 template <typename T,
-          enable_if_t<!std::is_constructible<std::string, T>::value && is_ostreamable<T>::value, detail::enabler> =
-              detail::dummy>
+          enable_if_t<!std::is_convertible<std::string, T>::value && !std::is_constructible<std::string, T>::value &&
+                          is_ostreamable<T>::value,
+                      detail::enabler> = detail::dummy>
 std::string to_string(T &&value) {
     std::stringstream stream;
     stream << value;

tests/AppTest.cpp

@@ -1885,6 +1885,27 @@ TEST_F(TApp, VectorUnlimString) {
     EXPECT_EQ(answer, strvec);
 }
 
+// From https://github.com/CLIUtils/CLI11/issues/420
+TEST_F(TApp, stringLikeTests) {
+    struct nType {
+        explicit nType(const std::string &a_value) : m_value{a_value} {}
+
+        explicit operator std::string() const { return std::string{"op str"}; }
+
+        std::string m_value;
+    };
+
+    nType m_type{"abc"};
+    app.add_option("--type", m_type, "type")->capture_default_str();
+    run();
+
+    EXPECT_EQ(app["--type"]->as<std::string>(), "op str");
+    args = {"--type", "bca"};
+    run();
+    EXPECT_EQ(std::string(m_type), "op str");
+    EXPECT_EQ(m_type.m_value, "bca");
+}
+
 TEST_F(TApp, VectorExpectedRange) {
     std::vector<std::string> strvec;