Issue #1258 (#1261)

fix an issue where an enumeration with a stream output method would
generate strings that could not be converted back to the original
enumeration value.

Fixes Issue #1258 

recent changes fixed a few issues with the default_val method. The
method used the to_string, which in cases where a user supplied a
streaming operation to enumerations they could not be converted back to
the enumeration properly resulting in some errors and confusing help
output.

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

include/CLI/ExtraValidators.hpp

@@ -358,7 +358,9 @@ class CheckedTransformer : public Validator {
             out += detail::generate_map(detail::smart_deref(mapping)) + " OR {";
             out += detail::join(
                 detail::smart_deref(mapping),
-                [](const iteration_type_t &v) { return detail::to_string(detail::pair_adaptor<element_t>::second(v)); },
+                [](const iteration_type_t &v) {
+                    return detail::value_string(detail::pair_adaptor<element_t>::second(v));
+                },
                 ",");
             out.push_back('}');
             return out;

include/CLI/Option.hpp

@@ -815,7 +815,7 @@ class Option : public OptionBase<Option> {
     /// Set the default value and validate the results and run the callback if appropriate to set the value into the
     /// bound value only available for types that can be converted to a string
     template <typename X> Option *default_val(const X &val) {
-        std::string val_str = detail::to_string(val);
+        std::string val_str = detail::value_string(val);
         auto old_option_state = current_option_state_;
         results_t old_results{std::move(results_)};
         results_.clear();

tests/TransformTest.cpp

@@ -186,6 +186,27 @@ TEST_CASE_METHOD(TApp, "SimpleTransformFn", "[transform]") {
     CHECK(1 == value);
 }
 
+enum class Color { kRed, kBlue };
+
+// operator<< outputs full enum name (standard practice)
+inline std::ostream &operator<<(std::ostream &os, Color c) { return os << (c == Color::kRed ? "kRed" : "kBlue"); }
+
+// test from https://github.com/CLIUtils/CLI11/issues/1258 [huweiATgithub](https://github.com/huweiATgithub)
+TEST_CASE_METHOD(TApp, "streamTransformCheck", "[transform]") {
+
+    std::map<std::string, Color> color_map = {
+        {"red", Color::kRed},   // User types "red"
+        {"blue", Color::kBlue}  // User types "blue"
+    };
+    Color color = Color::kRed;
+
+    app.add_option("--color", color)
+        ->transform(CLI::CheckedTransformer(color_map, CLI::ignore_case))
+        ->default_val(Color::kRed);  // BUG: Validates "kRed" against {"red", "blue"}
+
+    CHECK_NOTHROW(app.parse(""));  // Should use default
+}
+
 #if defined(CLI11_HAS_STRING_VIEW)
 TEST_CASE_METHOD(TApp, "StringViewTransformFn", "[transform]") {
     std::string value;