Allow rvalue references to be use with add_flag with no assignment

[tnixeu]

I am starting to use CLI11 in a project and I came across an inconvenience in code like this

const std::string description(std::format("description"));
app.add_flag("--flag", description);

As far as I can tell. I cannot directly use the description here. The reason seems to be to distinguish between assignment and description. So only const std::string or const char* and so on can be used here. But a const std::string && can not be used.

With the changes in the PR the code can also be written like this:

app.add_flag("--flag", std::format("description"));

The add_flag function can then be used with a const string or an rvalue reference to a string.

A bit OT: I was a bit confused by this part of the existing comment if a variable string is passed that variable will be assigned the results from the flag I think this might be wrong, but I am not sure.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (e4ee3af) to head (9c341f0).
Report is 105 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##              main     #1173     +/-   ##
===========================================
  Coverage   100.00%   100.00%             
===========================================
  Files           17        17             
  Lines         4546      5166    +620     
  Branches         0      1061   +1061     
===========================================
+ Hits          4546      5166    +620     

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[phlptp]

I was a bit confused by this part of the existing comment if a variable string is passed that variable will be assigned the results from the flag I think this might be wrong, but I am not sure.

This is basically saying that if you pass a non const string here, that is an assignment variable, so the results of parsing that flag would get passed into that variable. add_flag("--flag", variable, "description") so if the flag were used in the command line then the output would get put into variable.

[phlptp]

I like it, may add a few more tests later on, but this looks good, I am going to merge it

[tnixeu]

I was a bit confused by this part of the existing comment if a variable string is passed that variable will be assigned the results from the flag I think this might be wrong, but I am not sure.

This is basically saying that if you pass a non const string here, that is an assignment variable, so the results of parsing that flag would get passed into that variable. add_flag("--flag", variable, "description") so if the flag were used in the command line then the output would get put into variable.

I see, but in this case the std::enable_if would block it. It seems as if the comment is meant for another function.

[phlptp]

Yeah you are probably right, that comment is probably copied from the other overloads. I will clean that up in another PR soon.
Thanks