Make `std::variant<>` a soft error (SFINAE friendly)

[ilazaric]

https://godbolt.org/z/8xojM67ev
Simplified, without <variant> : https://godbolt.org/z/KdK8n76hb
Proposed alternative, on simplified: https://godbolt.org/z/PPrPzjfb1
So instead of the internal static_assert(), I suggest moving the condition into a constraint
The static_assert() in question: https://github.com/llvm/llvm-project/blob/main/libcxx/include/variant#L1164

[ilazaric]

Forgot passing -stdlib=libc++, updating description

[ilazaric]

After fixing the links, seems my fix doesn't work
Fix version with clang: https://godbolt.org/z/GxshffYsM (doesn't work)
Fix version with gcc: https://godbolt.org/z/zYrz5Tf8E (works)
Unsure what I think

[ilazaric]

This works though: https://godbolt.org/z/qWrf9G8j1
^ this is mimicking better the context in which I came across this
bla is std::cout, noop is std::endl

[philnik777]

This is how every implementation handles it (https://godbolt.org/z/hdh96essd) and also how the standard specifies it (http://eel.is/c++draft/variant#variant.general-3), so I don't think we should add an extension. Since that is our general policy anyways I'm closing this. If you want this kind of thing to work you can simply constrain your function instead.