Does `is` expression with template alias need fixing.

[FeepingCreature]

On Wednesday, 22 March 2023 at 07:25:04 UTC, Elfstone wrote:
> I wouldn't call what I want a new feature, but a fix to this 
> big hole in the language design (or implementation). People 
> would expect an alias works the same as the original symbol, at 
> least people need not care if they are using an alias from std 
> (`Regex`), as long as the compiler will generate a warning 
> where an alias might cause problems.

Well, an alias works *the same as the original symbol,* that's 
rather the problem. You can't infer the template instantiation 
that produced the alias, because the compiler doesn't see the 
alias after the fact, because "an alias member" is not a lexical 
member in the same way that "a templated struct" is.

My opinion is that we're several steps into an X-Y problem here, 
where what we *actually want* is to indicate that a value 
fulfills a certain constraint - "is a vector" - in a more 
freeform fashion than template constraints allow us to, without 
having to write lots of `isVector` type predicates, in a way 
that's legible to the typesystem. I think something like type 
traits or typeclasses would fulfill this need in a cleaner way 
and also make a lot of range code better.