Fix adding Opts with | to lvalue Parser

This is the recommended way of adding new Opts in our documentation
for using custom main, but we did not compile the code to see if it
works. We now compile the example as part of the BUILD_EXAMPLES
option.

Fixes #2787

examples/232-Cfg-CustomMain.cpp

@@ -0,0 +1,41 @@
+
+//              Copyright Catch2 Authors
+// Distributed under the Boost Software License, Version 1.0.
+//   (See accompanying file LICENSE.txt or copy at
+//        https://www.boost.org/LICENSE_1_0.txt)
+
+// SPDX-License-Identifier: BSL-1.0
+
+// 232-Cfg-CustomMain.cpp
+// Show how to use custom main and add a custom option to the CLI parser
+
+#include <catch2/catch_session.hpp>
+
+#include <iostream>
+
+int main(int argc, char** argv) {
+  Catch::Session session; // There must be exactly one instance
+
+  int height = 0; // Some user variable you want to be able to set
+
+  // Build a new parser on top of Catch2's
+  using namespace Catch::Clara;
+  auto cli
+    = session.cli()           // Get Catch2's command line parser
+    | Opt( height, "height" ) // bind variable to a new option, with a hint string
+         ["--height"]         // the option names it will respond to
+         ("how high?");       // description string for the help output
+
+  // Now pass the new composite back to Catch2 so it uses that
+  session.cli( cli );
+
+  // Let Catch2 (using Clara) parse the command line
+  int returnCode = session.applyCommandLine( argc, argv );
+  if( returnCode != 0 ) // Indicates a command line error
+      return returnCode;
+
+  // if set on the command line then 'height' is now set at this point
+  std::cout << "height: " << height << std::endl;
+
+  return session.run();
+}

examples/CMakeLists.txt

@@ -30,6 +30,7 @@ set( SOURCES_IDIOMATIC_EXAMPLES
     110-Fix-ClassFixture.cpp
     120-Bdd-ScenarioGivenWhenThen.cpp
     210-Evt-EventListeners.cpp
+    232-Cfg-CustomMain.cpp
     300-Gen-OwnGenerator.cpp
     301-Gen-MapTypeConversion.cpp
     302-Gen-Table.cpp

src/catch2/internal/catch_clara.hpp

@@ -673,7 +673,9 @@ namespace Catch {
 
             template <typename T>
             friend Parser operator|( Parser const& p, T&& rhs ) {
-                return Parser( p ) |= CATCH_FORWARD(rhs);
+                Parser temp( p );
+                temp |= rhs;
+                return temp;
             }
 
             template <typename T>