Hello!

I was thinking, when trying to do the following (I know it isn't possible for clear specified reason, I was just tinkering):

std::allocator<char&> my_allocator;

we get good error input, but still not particularly the easiest to read. A way to make the output error better is with requires(!is_reference_v<_Ty>) after the template.

This will be particularly helpful when this is done with, say, std::vector, as the errors spewed are way more complicated than with allocator, obviously.

Here is what I mean, on msvc v19.latest:
https://godbolt.org/z/n5dE76na9

A situation where this could happen is when someone is trying to create a vector of a variable's type and that variable is a ref. It'd make reading the error to understand what's going on more easy. Perhaps this could even be turned into a static assert and get a message alongside.

You can comment the require constraint to see the difference.

From what I understood talking with others, this could classify as a "Quality of Implementation" enhancement. I tried to implement it but I can't think of a common denominator to reduce the amount of code and I do not want to litter and use up CI time, so I thought that a way to contribute without making things worse is to perhaps bring attention to this.

Kind regards.