Re: [PHP-DEV] [Discussion] Revisiting typed closures

> I would find this very useful

Glad we agree it brings value.

> That said, restricting it to closures and not any callable is very limiting,
> particularly now that we have first-class callable syntax.
> Perhaps something more along the line of: callable(int):bool

I actually read first-class callables the other way -- as what makes
Closure-only *not* limiting: `strlen(...)`, `$obj->method(...)` and
`Foo::bar(...)`
all produce a Closure, so any callable can be passed by appending (...).

That is relevant, because this is a runtime check, not a static one. A Closure
is already resolved -- scope-bound, with its signature on the object, no lookup
and no side effects. A callable is not: 'foo' resolves against the caller's
namespace, and [Foo::class, 'bar'] must be resolved and possibly autoloaded
before its signature is even visible. Enforcing callable(int): bool at a
boundary would mean doing that resolution -- autoload included -- mid-call,
which is exactly the surprise I want to keep out of a type check. (callable also
cannot be a property type today, while Closure can.)