Merge branch 'master' into jgrpp

# Conflicts:
#	src/error.h
#	src/error_gui.cpp
#	src/linkgraph/linkgraph_gui.cpp
#	src/misc_gui.cpp
#	src/newgrf_gui.cpp
#	src/news_gui.cpp
#	src/rail_cmd.cpp
#	src/saveload/gamelog_sl.cpp
#	src/script/api/script_text.cpp
#	src/script/script_instance.cpp
#	src/statusbar_gui.cpp
#	src/strings.cpp
#	src/strings_func.h
#	src/strings_internal.h
#	src/table/settings/gui_settings.ini
#	src/table/settings/linkgraph_settings.ini
#	src/textbuf_gui.h

src/strings_type.h

@@ -10,6 +10,8 @@
 #ifndef STRINGS_TYPE_H
 #define STRINGS_TYPE_H
 
+#include <optional>
+
 /**
  * Numeric value that represents a string, independent of the selected language.
  */
@@ -92,4 +94,34 @@ enum SpecialStrings {
 	SPECSTR_TEMP_START         = 0x7000,
 };
 
+/** Data that is to be stored when backing up StringParameters. */
+struct StringParameterBackup {
+	uint64_t data; ///< The data field; valid *when* string has no value.
+	std::optional<std::string> string; ///< The string value.
+
+	/**
+	 * Assign the numeric data with the given value, while clearing the stored string.
+	 * @param data The new value of the data field.
+	 * @return This object.
+	 */
+	StringParameterBackup &operator=(uint64_t data)
+	{
+		this->string.reset();
+		this->data = data;
+		return *this;
+	}
+
+	/**
+	 * Assign a copy of the given string to the string field, while clearing the data field.
+	 * @param string The new value of the string.
+	 * @return This object.
+	 */
+	StringParameterBackup &operator=(const std::string_view string)
+	{
+		this->data = 0;
+		this->string.emplace(string);
+		return *this;
+	}
+};
+
 #endif /* STRINGS_TYPE_H */