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Technically, you're not supposed to add your own specialisations to the `std` namespace

In general this isn’t true (i guess it is in this specific context). For example I believe it’s totally expected to specialize std hash

I’ve also done this with hash… though given the footguns scattered about, I wouldn’t be surprised if it broke the spec.

That is a completely intended way to use std::hash, along with a few other functions like std::tuple_size and std::tuple_element.