make read_one_alternative more general

[ftschindler]

MSVC 17.14 did not pick up the move assignment here and was complaining about a missing copy assignment operator in std::optional. Using emplace fixes the issue for me.

I did not run the tests locally, please let mo know if you need any further information!

[gemini-code-assist]

Summary of Changes

Hello @ftschindler, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a compiler-specific issue encountered with MSVC 17.14, where the compiler was not correctly handling move assignment for std::optional, leading to a complaint about a missing copy assignment operator. The change modifies the read_one_alternative logic to use an emplace call instead of a direct move assignment when populating a std::optional result, thereby resolving the compilation error and improving compatibility across different compilers.

Highlights

• Compiler Compatibility Fix: Replaced a direct move assignment (*_result = std::move(*res);) with an emplace call (_result->emplace(std::variant<AlternativeTypes...>{std::move(*res)});) within the read_one_alternative method in Parser_variant.hpp to resolve a std::optional related compilation error specific to MSVC 17.14.

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[gemini-code-assist]

Code Review

The change from move assignment to emplace is a good way to work around the MSVC compiler bug. I've suggested a small improvement to make the emplace call more direct and idiomatic, which should also be slightly more performant by avoiding the creation of an explicit temporary std::variant.

[ftschindler]

Hi @liuzicheng1987 @Urfoex, I'm calling you out directly as you're listed in https://github.com/getml/reflect-cpp?tab=readme-ov-file#the-team-behind-reflect-cpp. I've never contributed to reflect-cpp before, what is your process for considering contributions?

[liuzicheng1987]

@ftschindler , if the checks pass, the contribution is something that we would be willing to maintain the code quality is good, we merge.

[liuzicheng1987]

@ftschindler , thanks for your contribution!

[ftschindler]

Great, thanks for accepting!