Clara option agglomeration does not work as before anymore with gnu++23

[AlexisWilke]

Describe the bug

As per this documentation I can add my own options in this way:

auto cli
    = session.cli()           // Get Catch2's command line parser
    | Opt( height, "height" ) // bind variable to a new option, with a hint string
        ["-g"]["--height"]    // the option names it will respond to
        ("how high?");        // description string for the help output

That does not compile anymore in 3.5.1. It used to work in 3.5.0.

In file included from /home/snapwebsites/snapcpp/BUILD/Debug/dist/include/catch2/internal/catch_commandline.hpp:11,
                 from /home/snapwebsites/snapcpp/BUILD/Debug/dist/include/catch2/catch_session.hpp:11,
                 from /home/snapwebsites/snapcpp/BUILD/Debug/dist/include/catch2/snapcatch2.hpp:29,
                 from /home/snapwebsites/snapcpp/contrib/libexcept/tests/catch_main.h:23,
                 from /home/snapwebsites/snapcpp/contrib/libexcept/tests/catch_main.cpp:25:
/home/snapwebsites/snapcpp/BUILD/Debug/dist/include/catch2/internal/catch_clara.hpp: In instantiation of ‘Catch::Clara::Parser Catch::Clara::operator|(const Catch::Clara::Parser&, T&&) [with T = Catch::Clara::Opt]’:
/home/snapwebsites/snapcpp/contrib/libexcept/tests/catch_main.cpp:59:115:   required from here
/home/snapwebsites/snapcpp/BUILD/Debug/dist/include/catch2/internal/catch_clara.hpp:676:24: error: cannot bind non-const lvalue reference of type ‘Catch::Clara::Parser&’ to an rvalue of type ‘Catch::Clara::Parser’
  676 |                 return Parser( p ) |= CATCH_FORWARD(rhs);
      |                        ^~~~~~~~~~~
/home/snapwebsites/snapcpp/BUILD/Debug/dist/include/catch2/internal/catch_clara.hpp:667:48: note:   initializing argument 1 of ‘Catch::Clara::Parser& Catch::Clara::operator|=(Catch::Clara::Parser&, Catch::Clara::Opt&&)’
  667 |             friend Parser& operator|=( Parser& p, Opt&& opt ) {
      |                                        ~~~~~~~~^

which says that the value on the left side needs to be a variable.

I could change the code this way to make it compile:

auto cli = session.cli()           // Get Catch2's command line parser
cli |= Opt( height, "height" ) // bind variable to a new option, with a hint string
        ["-g"]["--height"]    // the option names it will respond to
        ("how high?");        // description string for the help output

Expected behavior

No error compiling code.

Reproduction steps

See above.

Platform information:

• OS: Ubuntu 22.04.03
• Compiler+version: g++ (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0
• Catch version: v3.5.1

Additional context

I use the -std=g++23 command line option to compile my code.

[horenmar]

For the record, the exact standard does not have anything to do with the error, combining Parser const& with Opt would fail under any standard.

[johnhe4]

Thanks for the workaround @AlexisWilke. I have similar code that has been working for years but is now giving the same compilation error you posted, I'm guessing a brew upgrade was what triggered the issue for me. Separating the construction of the cli object from the operator |= seems better all around and compiles fine.

[johnhe4]

To add a bit more context, this page from the Android project is similarly incorrect and will mislead folks.