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Date: Wed, 1 Feb 2017 00:27:57 +0100
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Subject: Re: [PHP-DEV] PHP's support to contravariance and covariance
From: cmbecker69@gmx.de ("Christoph M. Becker")

On 31.01.2017 at 21:47, Levi Morrison wrote:

>> Is there anything else that I am missing?
> 
> Sadly, yes. Consider the following snippet:
> 
>     class A {
>         function method(): B;
>     }
> 
>     class B extends A {
>         function method(): C;
>     }
> 
>     class C extends B {}
> 
> When checking that B::method satisfies the requirements of A::method
> it must know if C extends B. At the time we check if B::method
> satisfies A::method, C will *not* yet be in the symbol table.
> 
> You need to adjust the passes over the code to register symbols and
> their declared relationships, and then in a separate pass validate
> them. After that if the class isn't found then you trigger an
> autoload.
> 
> It's doable, it just hasn't been done.

An alternative *might* be forward class declarations:

    class B extends A;
    class C extends B;

    class A {
        function method(): B;
    }

    class B extends A {
        function method(): C;
    }

    class C extends B {}

I haven't really thought about the feasibility – just throwing in a
rough idea.

-- 
Christoph M. Becker