🆕 Add mp_quote_expr to quote with lambda expressions

[krzysztof-jusiak]

Problem:

• There is no easy way to quote metafunction directly with lambda expressions.

Solution:

• Add mp_quote_expr.

Note:

• mp_quote_expr requires C++20.

[krzysztof-jusiak]

@pdimov would you be able to take a look, please?

[pdimov]

Do you have an example of actual use? The example you're adding to the docs doesn't quite do what it says in the description; it replaces all types not having value with void, instead of finding those that have value.

You need to check feature macros against their specified values (e.g. >= 2019xx), not just whether they are defined. (For a lambda to be usable as Expr the compiler also needs to support lambdas in unevaluated contexts, but I suppose the trait doesn't technically require that.)

[krzysztof-jusiak]

Thanks for your feedback, @pdimov.

Sure, I'll update the example to more descriptive. The basic idea is just to enable C++20 style lambdas (especially with if constexpr and requires) for simplicity of usage (instead of declaring aliases).

Yeah, I was trying that but both clang and gcc didn't set the date correctly.

[krzysztof-jusiak]

@pdimov Updated the example. Hopefully, the update makes it more clear/descriptive.
I also check again whether feature macros can be used purely without any workarounds but it seems that they always return the same standard date although the implementation differs :/

• https://godbolt.org/z/Ga9zx4Gs8

[krzysztof-jusiak]

@pdimov I updated feature macros to verify the date as you suggested. I hope that works for you. Is there anything else you would like me to address? Thank you.

[pdimov]

It's fine; but I have more pressing things to do at the moment, so I'll need to come back to this later, and maybe not in time for 1.77.

[krzysztof-jusiak]

No worries @pdimov , fully understood. Thank you. Pinging, just in case you are free and/or to show that the MR isn't stale.

[krzysztof-jusiak]

Hey @pdimov. I know you are busy, just pinging in case you are available, no worries if not. Thanks.

[pdimov]

I sat on this for a while because I wasn't sure what I want to do with it. Adding references doesn't feel like the right thing to do, for example.

But I've now made up my mind and I don't think it belongs in the library, it's not a good fit conceptually. Sorry.

It's still cool and would make a good blog post.