[SUGGESTION] unsafe_narrow is not a good name for forced upcasts.

[MaxSagebaum]

If you have e.g. expression templates, then the upcast to the implementing class needs to be done with unsafe_narrow.

Maybe an alias of unsafe_narrow like unsafe_cast, 'unsafe_pointer_cast` or something like that could be introduced.

Example: https://cpp2.godbolt.org/z/jb1MY3Mcq

Exp : <Impl> type = {  
  cast: (this) -> forward Impl = cpp2::unsafe_narrow<Impl>(this&)*; // unsafe_narrow does not really express what this cast is.
}

A : type = {
  this: Exp<A> = ();
  foo: (this) = std::cout << "foo" << std::endl;
}

main: () -> int = {
  a: A = ();
  e: *Exp<A> = a&;

  e*.cast().foo();

  return 0;
}

[JohelEGP]

I call this static_downcast: https://johelegp.github.io/jegp/static.downcast.
Inspired by https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Res-casts-named,
I created my own more specific one.

[MaxSagebaum]

Yes, that is also possible.

[JohelEGP]

By the way,
I noticed in your other issue (and this one)
that you mixed up the terms upcast and downcast.
Then, I tried to find it in the specification for static_cast,
but it wasn't mentioned, so I just left it unsaid.
Now, I did find other examples.
These use "upcast" when converting from derived to base:
https://eel.is/c++draft/class.cdtor#example-3 and https://eel.is/c++draft/class.cdtor#example-1.

[MaxSagebaum]

Thanks for pointing this out. I knew, that this can also be seen the other way round. I will start using it in terms of the type system and not the functionality.

[JohelEGP]

cpp2::unsafe_narrow is modeled after gsl::narrow_cast.
gsl::narrow_cast seems to be modeled after gsl::narrow
based on its uses in the C++ Core Guidelines.
Although gsl::narrow_cast<T>(x) is specified as being static_cast<T>(x)
[ Note: static_cast<T>(x) is illegal Cpp2. -- end note ]
it's not clear if those should work with non-arithmetic types:
See isocpp/CppCoreGuidelines#1845 and linked issues (and their linked issues/PRs).

[MaxSagebaum]

Ok, cpp2::unsafe_narrow should be equally handled as the gsl one. But there should be additional casts in cpp2.