Add is_subcommand_used overload

It's useful for removing string literals duplication in a code.

README.md

@@ -825,7 +825,7 @@ Subcommands:
 
 When a help message is requested from a subparser, only the help for that particular parser will be printed. The help message will not include parent parser or sibling parser messages.
 
-Additionally, every parser has a `.is_subcommand_used("<command_name>")` member function to check if a subcommand was used. 
+Additionally, every parser has the `.is_subcommand_used("<command_name>")` and `.is_subcommand_used(subparser)` member functions to check if a subcommand was used. 
 
 ### Parse Known Args
 

include/argparse/argparse.hpp

@@ -1255,13 +1255,18 @@ public:
     return (*this)[arg_name].m_is_used;
   }
 
-  /* Getter that returns true for user-supplied options. Returns false if not
+  /* Getter that returns true if a subcommand is used.
-   * user-supplied, even with a default value.
    */
   auto is_subcommand_used(std::string_view subcommand_name) const {
     return m_subparser_used.at(subcommand_name);
   }
 
+  /* Getter that returns true if a subcommand is used.
+   */
+  auto is_subcommand_used(const ArgumentParser &subparser) const {
+    return is_subcommand_used(subparser.m_program_name);
+  }
+
   /* Indexing operator. Return a reference to an Argument object
    * Used in conjuction with Argument.operator== e.g., parser["foo"] == true
    * @throws std::logic_error in case of an invalid argument name

test/test_subparsers.cpp

@@ -199,4 +199,41 @@ TEST_CASE("Parse git commands" * test_suite("subparsers")) {
     REQUIRE(submodule_update_command.get<bool>("--init") == true);
     REQUIRE(submodule_update_command.get<bool>("--recursive") == true);
   }
+}
+
+TEST_CASE("Check is_subcommand_used after parse" * test_suite("subparsers")) {
+  argparse::ArgumentParser command_1("add");
+
+  argparse::ArgumentParser command_2("clean");
+  command_2.add_argument("--fullclean")
+        .default_value(false)
+        .implicit_value(true);
+
+  argparse::ArgumentParser program("test");
+  program.add_subparser(command_1);
+  program.add_subparser(command_2);
+
+  SUBCASE("command 1") {
+    program.parse_args({"test", "add"});
+    REQUIRE(program.is_subcommand_used("add") == true);
+    REQUIRE(program.is_subcommand_used(command_1) == true);
+    REQUIRE(program.is_subcommand_used("clean") == false);
+    REQUIRE(program.is_subcommand_used(command_2) == false);
+  }
+
+  SUBCASE("command 2") {
+    program.parse_args({"test", "clean", "--fullclean"});
+    REQUIRE(program.is_subcommand_used("add") == false);
+    REQUIRE(program.is_subcommand_used(command_1) == false);
+    REQUIRE(program.is_subcommand_used("clean") == true);
+    REQUIRE(program.is_subcommand_used(command_2) == true);
+  }
+
+  SUBCASE("none") {
+    program.parse_args({"test"});
+    REQUIRE(program.is_subcommand_used("add") == false);
+    REQUIRE(program.is_subcommand_used(command_1) == false);
+    REQUIRE(program.is_subcommand_used("clean") == false);
+    REQUIRE(program.is_subcommand_used(command_2) == false);
+  }
 }