PHP: rfc:allow-constant-override-consistently

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====== PHP RFC - Consistent class-constant override ======

  * Version: 0.0.1
  * Date: 2018-04-11
  * Author: Pedro Magalhães, WesNetmo
  * Status: Under Discussion
  * First Published at: https://wiki.php.net/rfc/allow-constant-override-consistently

===== Introduction =====

Currently, a class constant can be overridden in inheritors, but it's disallowed in interfaces.

<code php>
// The following works:
class A1                { const X = 1; }
class B1 extends A1     { const X = 2; }

// This doesn't:
interface A2            { const X = 1; }
interface B2 extends A2 { const X = 2; }

// This doesn't work either:
interface A3            { const X = 1; }
class B3 implements A3  { const X = 2; }
</code>

Even if this restriction had a purpose, it's not really effective since it has only effect in direct inheritors:

<code php>
// This works:
interface A3                    { const X = 1; }
abstract class B3 implements A3 { } // Can't override here
class B4 extends B3             { const X = 2; } // But can do here
</code>

===== Proposal =====

This RFC proposes to normalize the behavior so that constant override is allowed everywhere. This is not an invite at reckless overriding but simply a matter of consistency. Class constants are **constants only within the class they are defined in**; child classes inherit them by default but the user can optionally override them if desired.

===== Backward Incompatible Changes =====

None.

===== Proposed PHP Version: =====

7.3

===== Future scope: =====

If desired by the community, the '''final''' keyword could be introduced in order to actually prevent constant override.

===== Voting =====

2/3 majority will be required.

===== References =====

- [[...........|Discussion on externals]]
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