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>

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;