Add decltype.

[Pentarctagon]

Already ran it on my fork's CI, so merging immediately.

[CelticMinstrel]

I'm kinda surprised that works, since decltype(evt) looks like it should be known at instantiation time. The ideal solution is probably to use an auto template argument in the definition:

template<auto>
inline constexpr bool dependent_false_v = false;

But I'm not sure if that actually works in C++17, and I'm also not sure if auto can deduce to typename. I think it should work on some version though, so it's something to keep in mind for the future…