Good point, this is a problem that const variables do not have. This reinforces the idea that const variables are not equivalent to const-function arguments (which are equivalent to const parameters). Having said this, the error messages for this do not need to mention structural equality or any other technical terms. const-generics is going to have these problems and if we can have good error messages there we can have them here as well. This is important because RFC2000 makes it very clear floating point values will never be usable in const generics because they are not reflexive, so I would guess that floating point types in particular will probably get their own custom error message because this is a common error to make.
@leonardo From the RFC2000:
This restriction can be analogized to the restriction on using type variables in types constructed in the body of functions - all of these declarations, though private to this item, must be independent of it, and do not have any of its parameters in scope.