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Date: Mon, 11 May 2026 02:07:02 +0100
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Subject: Re: [PHP-DEV] [RFC] [Discussion] Bound-Erased Generic Types
To: Bob Weinand <bobwei9@hotmail.com>
Cc: php internals <internals@lists.php.net>
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From: azjezz@carthage.software (Seifeddine Gmati)

Hi Bob,

>
> Thanks for the quick reply!
>
> Let me respond inline to avoid backtracking too much.
>
> On 11.5.2026 01:05:25, Seifeddine Gmati wrote:
>
> I have a bunch of questions and feedback:
>
> The requirement of ordering seems unnecessary to me - why would we not wa=
nt to be able to write <T: Box<U>, U: Box<T>>. Alternatingly recursive type=
s are not unheard of. Seems like an arbitrary restriction; and for compilat=
ion purposes it only requires collecting all parameter names before evaluat=
ing them.
>
> Your tests also show restrictions around intersection types, e.g. "Type p=
arameter T with bound mixed cannot be part of an intersection type" for 'cl=
ass Foo {} function x<T>(): T & Foo {}'. What's the motivation behind it? T=
his looks fairly natural to me: x() promises to return an instance of Foo w=
hich also fulfills the bound T. Any child class of Foo which happens to imp=
lement T will fulfill that contract.
>
> I would like to plead to skip the arity validation, except for "more para=
meters than allowed":
> - This inhibits graceful addition of generics - any library adding them r=
equires callers to immediately update all caller sites.
>   - It would also make addition of generics to Iterator classes etc. comp=
letely uncontroversial.
> - This would be more in line with PHP's general "no type is effectively t=
he highest possible bound" approach. I.e. "class A extends Box" and "class =
A extends Box<mixed>" would be equivalent.
> - This would also allow for future incremental runtime generics: you'd st=
art with <never> and as you call stuff with values, the type becomes broade=
r.
>
>
> This is the one thing which makes the whole RFC a non-starter for me if r=
equired:
>
> Typing is optional in PHP!
>
>
> Your tests show that this specific example is allowed, which strikes me a=
s odd. Why would we not check the arity here?
>
> class Container {}
> function f(Container<int> $x): Container<string> { return $x; }
>
>
> Diamond checks:
>
> Are these necessarily problematic? if you inherit Box<int> and Box<string=
>, it simply means that the generic parameter, when placed in a contravaria=
nt location will accept int|string, when placed into return or property typ=
es it'll evaluate to never.
>
> If you disagree (that's possibly fine), a diamond covariant parameter sho=
uld be allowed in any case though, i.e. if Box<+T>, then an interface shall=
 be able to implement Box<string>, Box<int>. At least at a glance I don't f=
ind such a test - if it already works, nice, then please just add the test!
>
>
> Is class ABox implements Box<self> allowed, or do we need to write implem=
ents Box<ABox>?
>
>
> I'm also not sold on the turbofish syntax. I hate it in Rust, which I hav=
e to write nearly daily. I forget these :: SO often. And then the Linter ye=
lls at me and I correct it.
> I understand that there are language limitations, in particular with the =
array syntax, but honestly, I'd rather just have the parser shift in favor =
of the existing syntax - for these rare conflicting cases forcing parenthes=
is around the generic would be nicer, i.e. `[A<B, B>(C)]` would continue ca=
rrying the meaning it has today, and we'd require writing `[(A<B, B>(C))]` =
for that case.
>
>
> I'm not quite sure if + and - are the proper choices. I'm more used to C#=
 myself with in and out being more obvious to me. I also admit that I initi=
ally assumed "+" to be covariant - the sum of stuff accepted, and "-" contr=
avariant, subtracting what can be returned. But this particular bikesheds c=
olor is not too important to me.
>
>
> Otherwise, it's a pretty solid RFC which should be extensible with runtim=
e generics eventually. (In particular runtime generics on the class inherit=
ance level should be a no-brainer to add with the existing syntax.)
>
>
> Thanks,
> Bob
>
> Thanks for the careful read. Going point by point.
>
> 1. Ordering of type parameter declarations
>
> The restriction is implementation-level, not fundamental. We register
> parameter names before we compile bounds, so allowing <T: Box<U>, U:
> Box<T>> is a "small" change. I left it out for the initial cut because
> I didn't want to bake mutually-recursive bounds into the spec without
> seeing whether anyone actually wants them in practice. If others agree
> this is worth having, I'm happy to drop the restriction before vote.
>
> Yes, that's the impression I had - an arbitrary restriction to make it a =
bit simpler at compile time.
>
> I'd suggest just dropping it, why have it, actually? It should be a relat=
ively easy change. I don't see any concrete advantage of this, apart from t=
he minor simplification this restriction would have in compiler.
>
> 2. Type parameters in intersection types
>
> The check rejects an intersection where one side is a type parameter
> whose bound is `mixed`, because the erased form can be anything,
> including a scalar. Scalars don't intersect with anything, today. (
> ref https://3v4l.org/mdvFA#v )
>
> The error message in the test you saw is precisely about the unbounded
> case. If `T` is bound to an object-shaped type (`T: object`, `T:
> SomeInterface`, `T: SomeClass`, ...), then `T & Foo` is allowed. the
> erased form is guaranteed to be a legal intersection operand. So this
> is the same rule PHP already enforces today, just applied through the
> erased form.
>
> Ah, I see, it needs a T: object. (or named class). It's not quite obvious=
 from the error message, so I'd suggest adding a suggestion for "at least T=
: object or a stronger bound" then.
>
> That makes some sense. The question would be if never types should be pos=
sible to reached, but this I've basically asked already when asking about d=
iamond checks.
>
> 3. Arity validation at consumer call sites
>
> I think this one is a misunderstanding. Arity validation only fires
> when the caller writes turbofish. Without turbofish, nothing changes
> at the call site:
>
> ```
> function id<T>(T $v): T { return $v; }
>
> id($x);                 // no validation, no behavior change
> id::<int>($x);       // arity + bound checked
> ```
>
> So a library can add generic parameters to its public surface and
> every existing caller (none of which uses turbofish, because turbofish
> doesn't exist today) keeps working unchanged. The validation is opt-in
> at the use site. Same for `new` and method calls.
>
> This is exactly the graceful-addition story you're asking for. The
> existing tests demonstrate it.
>
> This is not quite obvious from the RFC. I'd recommend adding a subsection=
 to "What is enforced where" detailing that these are *not* checked: I thou=
ght "turbofish arity" would apply to everywhere, not just explicitly where =
the ::<> syntax is actually used.
>
> Are they also not checked for inheritance? Or just for caller sites?
>
> Sorry for missing it in tests, you have a LOT of tests!
>
> 4. Generic args on a non-generic class in a signature
>
> ```
> class Container {}
> function f(Container<int> $x): Container<string> { return $x; }
> ```
>
> This is accepted, and on purpose. PHP doesn't load classes from
> signatures, they load on use: https://3v4l.org/DnIKQ#v
>
> To validate arity at compile time, we'd have to load `Container`,
> which is a behavioral and performance regression. The cost of being
> strict here is much higher than the cost of being permissive. The same
> logic that already lets you reference an unloaded class in a signature
> lets you reference an unloaded class with type arguments in a
> signature. Validation happens once the class actually gets resolved at
> a use site (new, turbofish call, etc.).
>
> I'm actually suggesting validation at runtime here, i.e. once the class t=
ype check passes, to check whether the arity is matching for the class of t=
he argument.
>
> I'm certainly not asking for compile time checks here. But leaving this u=
nchecked sort-of makes it the odd-one out here.
>
> 5. Diamond inheritance
>
> The diamond check is necessary because methods get substituted with
> the type arguments at link time. Consider:
>
> ```
> interface Box<T> { public function set(T $v): void; }
>
> class C implements Box<int>, Box<string> {}
> ```
>
> After substitution, C must implement both `set(int): void` and
> `set(string): void`. PHP has no way to represent two methods with the
> same name and different signatures ( i.e overloading ), one of them
> has to win, and either choice silently breaks one of the parent
> contracts. Same problem in contravariant position. The check rejects
> this at link time rather than letting it produce a class that violates
> its own interface.
>
> For purely covariant slots you have a point, `get(): int` and `get():
> string` could in principle be reconciled to `get(): int|string` (an
> LUB). The current implementation rejects all diamonds uniformly to
> keep linking deterministic and to avoid synthesizing union types
> during inheritance. Relaxing it for the covariant case is a reasonable
> follow-up, not something I want to bake in before vote.
>
> You got it the wrong way round, the union needs to be allowed on the para=
meters, not the return type.
>
> set(string): void and set(int): void can be merged into set(string|int): =
void.
>
> I'd also like to mention here that:
>
> interface A { public function set(int $v): void; }
> interface B { public function set(string $v): void; }
> class C implements A, B { public function set(int|string $v): void {} }
>
> is perfectly valid today. Not allowing this for the contravariant case wo=
uld make it inconsistent with what's currently supported in PHP.
>
> This needs no overloading at all.
>
> 6. `class ABox implements Box<self>`
>
> It is allowed and works as you'd expect. `self` resolves to the
> implementing class.
>
> ```
> interface Box<+T> { public function get(): T; }
>
> class ABox implements Box<self> {
>     public function get(): self { return $this; }
> }
>
> var_dump((new ABox)->get() instanceof ABox); // true
> ```
>
> Nice!
>
>
> 7. Turbofish
>
> We have to disagree here. Turbofish:
>   - has zero parser conflict with comparison operators in expression posi=
tion
>   - is uniform across `new`, function calls, method calls, FCCs, attribut=
es
>   - requires no context-sensitive disambiguation rule
>
> The alternative adds a rule a developer has to learn and apply at
> exactly the worst places (inside attributes, array expressions,
> ternaries). I'd rather pay the `::` tax than introduce a
> context-sensitive parser rule that bites people inside attributes
> specifically. Rust's choice was a forced one because of `<>` overload,
> and it's the right one for PHP too for the same reason.
>
> Alright, let's disagree here.
>
>
> 8. + / - markers
>
> Picked because they don't require any new reserved words. `in`/`out`
> reads well but I'm not comfortable burning two keywords for a feature
> where two pieces of punctuation already do the job.
>
> On the "+ =3D sum of accepted" intuition: the convention here is the
> standard one from variance literature. `+` marks positions where the
> type can be widened (covariant, e.g., returns), `-` marks positions
> where it can be narrowed (contravariant, e.g., parameters). It also
> matches Hack, Scala, and Kotlin, so there is prior art the ecosystem
> already maps to.
>
> I understand, I've never used any of those languages for more than target=
ed edits, so I didn't know.
>
> I guess it's fine to not diverge here.
>
> By the way, you don't necessarily need a new keyword, in fact you could j=
ust allow two consecutive T_STRING at that position and emit a parser error=
 when the first one is neither of "in" or "out".
>
>
> Thanks,
> Bob


Thanks for the reply!

1. Ordering of type parameter declarations

Agreed, dropped. Forward references and mutually recursive bounds
within a single parameter list are now allowed:

```
function f<U : T, T>(U $x): T { /* ... */ }            // forward
class Pair<T : Box<U>, U : Box<T>> {}                  // mutual
```

Defaults still require backward-only references, meaning omitted
arguments resolve in one pass at instantiation. Direct self-reference
at the head of a bound (`<T : T>`) is still rejected; the indirect
form (`<T : Box<T>>`) is still allowed.

Diff: https://github.com/php/php-src/compare/9ebcf28cef5563a63fe0bcc2fd5ec4=
5211fa1f15..f8ced4dacacda2038118bcc889f4905a92cf05de

2. Intersection error message

Improved the diagnostic to point directly at the fix instead of just
stating the rule. The message now reads:
```
Type parameter T with bound mixed cannot be part of an intersection
type; use an object-shaped bound (e.g. T: object)
```

Diff:
Thanks for the reply!

1. Ordering of type parameter declarations

Agreed, dropped. Forward references and mutually recursive bounds
within a single parameter list are now allowed:

```
function f<U : T, T>(U $x): T { /* ... */ }            // forward
class Pair<T : Box<U>, U : Box<T>> {}                  // mutual
```

Defaults still require backward-only references, meaning omitted
arguments resolve in one pass at instantiation. Direct self-reference
at the head of a bound (`<T : T>`) is still rejected; the indirect
form (`<T : Box<T>>`) is still allowed.

Diff: https://github.com/php/php-src/compare/9ebcf28cef5563a63fe0bcc2fd5ec4=
5211fa1f15..f8ced4dacacda2038118bcc889f4905a92cf05de

2. Intersection error message

Improved the diagnostic to point directly at the fix instead of just
stating the rule. The message now reads:
```
Type parameter T with bound mixed cannot be part of an intersection
type; use an object-shaped bound (e.g. T: object)
```

Diff: https://github.com/php/php-src/compare/f8ced4dacacda2038118bcc889f490=
5a92cf05de..6b5588d8e927d60e4b4509658af62601dfa0802a

3. Arity validation at consumer call sites

You're right that the RFC didn't say this clearly. Added a "What is
not checked" subsection under "What is enforced where" that lists the
exact sites where the engine intentionally omits arity or bounds
validation. The whole point of opt-in is the graceful-addition story
you're after; the RFC now spells that out.

4. Runtime arity check at call boundaries

In principle, sure. Once the runtime confirms the value matches the
class, we can also validate the signature's type arguments against the
class's actual declared arity and bounds. So in:

```
class C {}
function foo(C<int, string> $x): void {}

foo(new C());
```

we'd error because `C` has no generic parameters but the signature
supplied two type arguments.

The catch is that this isn't only about parameters. It applies to
every place the engine resolves a class-typed type expression at
runtime.

So a bit more complicated, not a small change. I want to spend more
time on it before committing to text: what exactly gets validated,
where the result gets cached so we aren't paying for it on every typed
call, how it interacts with the substitution chain at link time, and
what the hot-path cost actually is on a profiled workload. I'd like to
see what others on the list think too, since the call is a trade-off
between strictness and performance and people will weigh those
differently.

If after that the answer is "yes, fold it in", I'll fold it in. But I
don't want to promise it inside this RFC until I've done the
investigation.

5. Diamond inheritance - I had the direction wrong

Yea, sorry. Contravariant (parameter) positions are the ones that
merge cleanly into a union, not return positions. Your example is
right.

For the generic case, the contravariant side is the easy one:
```
interface Box<-T> { public function set(T $v): void; }

class C implements Box<int>, Box<string> { /* set(int|string) */ }
```

The implementer's substituted prototype is the union of the two
contravariant slots.

The covariant side is more nuanced. `get(): int` and `get(): string`
merged would have to return both ( i.e. `int & string` ), and PHP
rejects intersections involving scalars (because impossible!). So a
covariant diamond with scalar bindings is unrepresentable. It only
becomes representable when the type parameter is bounded by an
object-shaped type, in which case the implementer's return type
collapses cleanly to an intersection:

```
interface Box<+T : object> { public function get(): T; }
interface A {}
interface B {}

class C implements Box<A>, Box<B> {
    public function get(): A&B { /* ... */ }
}
```

Here `A & B` is a valid PHP intersection, so the merge is sound.

I'll look into this. I think we can fit it into this RFC, but I want
to investigate the implementation first. I keep the RFC and the
implementation in sync and don't want to commit to text that isn't
backed by working code yet.

Thanks,
Seifeddine.

3. Arity validation at consumer call sites

You're right that the RFC didn't say this clearly. Added a "What is
not checked" subsection under "What is enforced where" that lists the
exact sites where the engine intentionally omits arity or bounds
validation. The whole point of opt-in is the graceful-addition story
you're after; the RFC now spells that out.

4. Runtime arity check at call boundaries

In principle, sure. Once the runtime confirms the value matches the
class, we can also validate the signature's type arguments against the
class's actual declared arity and bounds. So in:

```
class C {}
function foo(C<int, string> $x): void {}

foo(new C());
```

we'd error because `C` has no generic parameters but the signature
supplied two type arguments.

The catch is that this isn't only about parameters. It applies to
every place the engine resolves a class-typed type expression at
runtime.

So a bit more complicated, not a small change. I want to spend more
time on it before committing to text: what exactly gets validated,
where the result gets cached so we aren't paying for it on every typed
call, how it interacts with the substitution chain at link time, and
what the hot-path cost actually is on a profiled workload. I'd like to
see what others on the list think too, since the call is a trade-off
between strictness and performance and people will weigh those
differently.

If the answer after that is "yes, add it,", I'll fold it in. But I
don't want to promise it inside this RFC until I've done the
investigation.

5. Diamond inheritance

Yea, sorry. Contravariant (parameter) positions are the ones that
merge cleanly into a union, not return positions. Your example is
right.

For the generic case, the contravariant side is the easy one:
```
interface Box<-T> { public function set(T $v): void; }

class C implements Box<int>, Box<string> { /* set(int|string) */ }
```

The implementer's substituted prototype is the union of the two
contravariant slots. No new type-system rules are needed.

The covariant side is more nuanced. `get(): int` and `get(): string`
merged would have to return both ( i.e. `int & string` ), and PHP
rejects intersections involving scalars (because impossible!). So a
covariant diamond with scalar bindings is unrepresentable. It only
becomes representable when the type parameter is bounded by an
object-shaped type, in which case the implementer's return type
collapses cleanly to an intersection:

```
interface Box<+T : object> { public function get(): T; }
interface A {}
interface B {}

class C implements Box<A>, Box<B> {
    public function get(): A&B { /* ... */ }
}
```

Here `A & B` is a valid PHP intersection, so the merge is sound.

I'll look into this. I think we can fit it into this RFC, but I want
to investigate the implementation first. I keep the RFC and the
implementation in sync and don't want to commit to text that isn't
backed by working code yet.

Thanks,
Seifeddine.