storage: fix UB taking pointer to inner type T

[johannst]

Casting the pointer Storage<T>::value_ to T is undefined according to the standard [basic.life/8], as it violates item (8.2), since the types are not equal [1].

Therefore, we need to launder the pointer after reinterpreting it.

See also the examples in std::launder[2] and std::aligned_storage [3].

Not sure I follow your style correctly, please let me know.

Best

[1] https://timsong-cpp.github.io/cppwp/n4868/basic.life#8
[2] https://en.cppreference.com/w/cpp/utility/launder
[3] https://en.cppreference.com/w/cpp/types/aligned_storage
[4] https://timsong-cpp.github.io/cppwp/n4868/ptr.launder

[TheNumbat]

Good timing, I was just thinking I should add a std::launder equivalent. I'll also add the launder in Variant.
Style looks fine (though I'm not sure how the existing code ended up using protected...)

[TheNumbat]

Oh, it seems MSVC doesn't expose __is_function. Will also fix that.

[johannst]

Forgot the poor variant again, out of sight out of mind I guess :^)

Style looks fine (though I'm not sure how the existing code ended up using protected...)

In the Storage class? If by accident, then we can also mark it private.

Oh, it seems MSVC doesn't expose __is_function. Will also fix that.

Oops, didn't have a machine at hand. Probably should have checked before. Thanks for fixing.

[TheNumbat]

Yeah, I just got rid of the protected.
Normally the CI build would catch errors from MSVC, but the GitHub runners don't have the latest compiler version yet.

[johannst]

A small note, __is_void() also checks for cv qualified void, whereas the new Is_Void trait does not.

[TheNumbat]

Ah, good point