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Date: Mon, 11 May 2026 08:17:49 +0100
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Subject: Re: [PHP-DEV] [RFC] [Discussion] Bound-Erased Generic Types
To: Levi Morrison <levim@php.net>
Cc: php internals <internals@lists.php.net>
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From: azjezz@carthage.software (Seifeddine Gmati)

>
> On Sun, May 10, 2026 at 1:05=E2=80=AFPM Seifeddine Gmati
> <azjezz@carthage.software> wrote:
> >
> > Hello Internals,
> >
> > I'd like to start the discussion on a new RFC adding bound-erased
> > generics types to PHP.
> >
> > Generic type parameters can be declared on classes, interfaces,
> > traits, functions, methods, closures, and arrow functions, with
> > bounds, defaults, and variance markers. Type parameters erase to their
> > bound at runtime; the pre-erasure form is preserved for Reflection and
> > consumed by static analyzers.
> >
> > - RFC: https://wiki.php.net/rfc/bound_erased_generic_types
> > - Implementation: https://github.com/php/php-src/pull/21969
> >
> > Thanks,
> > Seifeddine.
>
> Seifeddine,
>
> This is a very interesting RFC, thank you!
>
> I have a technical request: can we lower the limit from 255 to a 7-bit
> max? I've done tons of optimization work in the last 8-10 years of my
> life, and having a spare bit on things for the future has often been
> rewarded. And for me, I can't imagine a use-case for having 128-255
> type arguments in practice. Do you have any evidence that a 7-bit
> maximum would be insufficient for real-world code?
>
> Thanks, Levi Morrison

Hi Levi,

Good catch, lowered. The cap is now 127 (u7), with the high bit
reserved for future per-site metadata. RFC updated:
https://wiki.php.net/rfc/bound_erased_generic_types#the_127-argument_cap

Honestly, there is no real-world evidence either way, no PHP codebase
approaches either 127 or 255 type parameters, so this is purely about
reserving headroom.

Thanks,
Seifeddine.