Add a ArgumentParser::add_hidden_alias_for() method

It is sometimes desirable to offer an alias for an argument, but without it
appearing it in the usage. For example, to phase out a deprecated wording of
an argument while not breaking backwards compatible. This can be done with
the ``ArgumentParser::add_hidden_alias_for()` method.

```cpp
argparse::ArgumentParser program("test");

auto &arg = program.add_argument("--suppress").flag();
program.add_hidden_alias_for(arg, "--supress"); // old misspelled alias
```

README.md

@@ -38,6 +38,7 @@
      *    [Parent Parsers](#parent-parsers)
      *    [Subcommands](#subcommands)
      *    [Parse Known Args](#parse-known-args)
+     *    [Hidden alias](#hidden-alias)
      *    [ArgumentParser in bool Context](#argumentparser-in-bool-context)
      *    [Custom Prefix Characters](#custom-prefix-characters)
      *    [Custom Assignment Characters](#custom-assignment-characters)
@@ -970,6 +971,20 @@ int main(int argc, char *argv[]) {
 }
 ```
 
+### Hidden alias
+
+It is sometimes desirable to offer an alias for an argument, but without it
+appearing it in the usage. For example, to phase out a deprecated wording of
+an argument while not breaking backwards compatible. This can be done with
+the ``ArgumentParser::add_hidden_alias_for()` method.
+
+```cpp
+argparse::ArgumentParser program("test");
+
+auto &arg = program.add_argument("--suppress").flag();
+program.add_hidden_alias_for(arg, "--supress"); // old misspelled alias
+```
+
 ### ArgumentParser in bool Context
 
 An `ArgumentParser` is `false` until it (or one of its subparsers) have extracted

include/argparse/argparse.hpp

@@ -1578,6 +1578,21 @@ public:
     return *this;
   }
 
+  // Add a un-documented/hidden alias for an argument.
+  // Ideally we'd want this to be a method of Argument, but Argument
+  // does not own its owing ArgumentParser.
+  ArgumentParser &add_hidden_alias_for(Argument &arg, std::string_view alias) {
+    for (auto it = m_optional_arguments.begin();
+         it != m_optional_arguments.end(); ++it) {
+      if (&(*it) == &arg) {
+        m_argument_map.insert_or_assign(std::string(alias), it);
+        return *this;
+      }
+    }
+    throw std::logic_error(
+        "Argument is not an optional argument of this parser");
+  }
+
   /* Getter for arguments and subparsers.
    * @throws std::logic_error in case of an invalid argument or subparser name
    */

test/CMakeLists.txt

@@ -55,6 +55,7 @@ file(GLOB ARGPARSE_TEST_SOURCES
     test_parse_known_args.cpp
     test_equals_form.cpp
     test_prefix_chars.cpp
+    test_hidden_alias.cpp
 )
 set_source_files_properties(main.cpp
     PROPERTIES

test/test_hidden_alias.cpp

@@ -0,0 +1,18 @@
+#ifdef WITH_MODULE
+import argparse;
+#else
+#include <argparse/argparse.hpp>
+#endif
+#include <doctest.hpp>
+
+using doctest::test_suite;
+
+TEST_CASE("Test setting a hidden alias for an argument" *
+          test_suite("hidden_alias")) {
+  argparse::ArgumentParser program("test");
+  auto &arg = program.add_argument("--suppress").flag();
+  program.add_hidden_alias_for(arg, "--supress"); // old misspelled alias
+
+  program.parse_args({"./test.exe", "--supress"});
+  REQUIRE(program.get<bool>("--suppress") == true);
+}