Add StringMaker for std::variant, std::monostate (#1380)

The StringMaker is off by default and can be enabled by a new macro `CATCH_CONFIG_ENABLE_VARIANT_STRINGMAKER`, to avoid increasing the footprint of stringification machinery by default.

include/internal/catch_tostring.h

@@ -354,6 +354,7 @@ namespace Catch {
 #if defined(CATCH_CONFIG_ENABLE_ALL_STRINGMAKERS)
 #  define CATCH_CONFIG_ENABLE_PAIR_STRINGMAKER
 #  define CATCH_CONFIG_ENABLE_TUPLE_STRINGMAKER
+#  define CATCH_CONFIG_ENABLE_VARIANT_STRINGMAKER
 #  define CATCH_CONFIG_ENABLE_CHRONO_STRINGMAKER
 #endif
 
@@ -418,6 +419,34 @@ namespace Catch {
 }
 #endif // CATCH_CONFIG_ENABLE_TUPLE_STRINGMAKER
 
+#if defined(CATCH_CONFIG_ENABLE_VARIANT_STRINGMAKER) && defined(CATCH_CONFIG_CPP17_VARIANT)
+#include <variant>
+namespace Catch {
+    template<>
+    struct StringMaker<std::monostate> {
+        static std::string convert(const std::monostate&) {
+            return "{ }";
+        }
+    };
+
+    template<typename... Elements>
+    struct StringMaker<std::variant<Elements...>> {
+        static std::string convert(const std::variant<Elements...>& variant) {
+            if (variant.valueless_by_exception()) {
+                return "{valueless variant}";
+            } else {
+                return std::visit(
+                    [](const auto& value) {
+                        return ::Catch::Detail::stringify(value);
+                    },
+                    variant
+                );
+            }
+        }
+    };
+}
+#endif // CATCH_CONFIG_ENABLE_VARIANT_STRINGMAKER
+
 namespace Catch {
     struct not_this_one {}; // Tag type for detecting which begin/ end are being selected