[RFC] [HHVM] better reified generics support

[azjezz]

Standalone code, or other way to reproduce the problem

<<__ConsistentConstruct>>
final class A<<<__Newable>> reify T> {
  public function __construct(
    public classname<T> $class_name = T::class
  ) {}
}

<<__ConsistentConstruct>>
class B<reify T> {}
<<__ConsistentConstruct>>
final class C<reify Ti, reify To> extends B<Ti> {}

<<__EntryPoint>>
function main(): void {
  $a = create<A<C<B<float>, string>>>();

  print $a->class_name; // result: C | expected: C<B<float>, string>>
}

function create<<<__Enforcable, __Newable>> reify T>(): T {
  var_dump(T::class); // result: A | expected: A<C<B<float>, string>>>

  return new T();
}

Steps to reproduce the behavior:

1. write down the previous code in a .hack file
2. execute the file noramlly using hhvm

Expected behavior

output:

string(23) "A<C<B<float>, string>>>"
C<B<float>, string>>

Actual behavior

string(1) "A"
C

Environment

• Operating system irrevelent
• Installation method irrevelent
• HHVM Version - latest

Please include the output of hhvm --version and hh_client --version

Additional context

currently T::class returns classname<T>, hack has both typename<T> and classname<T> types, both these types are nothing more of a string at runtime, what would be a great solution is to make them not string, but rather something else ( e.g: simiar to function reference: class reference and type reference - which class reference being a subtype of type reference ), which can be used as is now, but cannot be used as a string, this new type can hold all generic parameter types, and optionally convereted to a string representation.

[lexidor]

Just adding to the pool of knowledge. The runtime representation of classname is currently a plain string indeed. LazyClass might become a new runtime representation of a classname. But afaik this does not intend to include reifyied generics.

856057b is where LazyClass was first mentioned.

It might be worth asking around what the idea is behind this type.

---

Could you please update your issue text with the hhvm version?

Your issue title is a bit generic. Would a variation of: "Can't retrieve reifyied generics from a classname<T>" describe your issue?

What happens when you use the new operator on a classname of a class with a reifyied generic?

[lexidor]

I am going over old issues on this repository, to see which ones apply to the current versions of hhvm.

classname<T> string forms do not contain generic information. The HH\ReifiedGenerics\get_typestructure<T>() function exposes information about inner generics. You are able to reconstruct the string name fully from the type structure.