> 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.)
