> Supposing that double is IEEE-754 binary64 and ptr has type double*,
> is it safe to reinterpret_cast<std::float64_t*>(ptr) and then use it to read or modify memory
> that is an array of doubles?
> I could not find that this is allowed, so I would like to float the idea for such a proposal.

Unless otherwise stated, this violates strict aliasing and it's not
allowed. Even if the compiler manual says that you can do it, it's
undefined behavior and wouldn't work in constant expressions.

We arguably have the right tools to deal with this situation already.
For scalars that you want to pass between libraries, you can
std::bit_cast<double> or std::bit_cast<std::float64_t> at the library
bounds. For blocks of mutable data, you can use
std::start_lifetime_as_array to reinterpret the elements without
accessing storage.

The only problem that we have no solution for, and this also comes up
a lot at library bounds where std::string_view/std::u8string_view are
used, is the ability to reinterpret immutable data with any other
type. If we were able to do that, this would break optimizations
though since any write to a char could change the value of a char8_t,
and any write to a double could change a std::float64_t, and vice
versa.