====== PHP RFC: instanceof improvements ======
  * Version: 1.0
  * Date: 2020-05-17
  * Author: Max Semenik, maxsem.wiki@gmail.com
  * Status: Withdrawn
  * First Published at: http://wiki.php.net/rfc/instanceof_improvements

===== Introduction =====
As currently implemented, the ''instanceof'' operator is inconsistent.
<code php>
var_dump(new MyClass instanceof MyClass); // true
</code>
So far so good? What if we tried to use a scalar type? They can be specified as parameter types just like class names, right?
<code php>
$x = 123;
var_dump($x instanceof int); // false, right hand side is always treated as a class name
</code>

===== Proposal =====
Make ''instanceof'' support non-object types:
<code php>
var_dump('foo' instanceof string);      // true
var_dump('foo' instanceof ('string'));  // true
$type = 'string';
var_dump('foo' instanceof $type);       // true
</code>

==== Types to support ====
This proposal covers only concrete scalar types ''int'', ''float'', ''string'', ''bool'' and ''null''; as well as compound types ''array'', ''object'', ''callable'' and ''iterable''. Other types are intentionally omitted:
  * ''mixed'' is pointless because there are easier ways to produce an expression always evaluating to true than ''$something instanceof mixed''.
  * ''void'' is kinda obvious, but I'm mentioning it just to be thorough.

All attempts to check against these types would evaluate to ''false'' (just as currently) and produce warnings in 8.1, upgraded to fatals in 8.2.

  * ''resource'' is not available as a parameter type and is on its way out, so it will be unaffected by this RFC and treated as a class name, available for userspace to use.

==== Legacy type aliases ====
There are several legacy types, supported only for typecasts: ''(integer)'', ''(double)'', ''(boolean)'' and ''(binary)''. They have never worked for any other situations: e.g. the first parameter of ''function f(integer $x)'' is interpreted as a class called ''integer'', we even started issuing warnings in such situations since 8.0. I propose to extend this kind of treatment to ''instanceof'' too, issuing the same warning.

==== Constant expressions on the left hand side ====
The current implementation has a shortcut where if there is a constant expression to the left of ''instanceof'', the result is hardcoded as false, since the operator supports only class names on the right and constant expressions can't produce objects. This raises a question: how should be Captain Obvious cases like ''123 instanceof int'' be treated? What about slightly less obvious cases like ''123 instanceof $typeName''? For comparison, I've checked two languages that have approaches to OOP similar to PHP:
  * In Java, a variable is required on the left side of ''instanceof''. All types of expressions are disallowed.
  * In C#, constant expressions to the left of ''is'' produce a correct result but a compiler warning is issued.
  * None of these support type names as strings, so the latter use case has no direct analogs.

I can see two possibilities why such constructs might appear in code:
  * An clueless developer trying to achieve with ''123 instanceof $type'' something for which ''$type === "int"'' is a better solution.
  * A code generator went astray and generates something dubious.

Considering this, I don't think that adding support for constant expressions on LHS would do our end users any good. I propose to continue shortcutting such cases to false (just to make sure they don't rely on this) and additionally let the developers know they're doing something wrong with ''E_COMPILE_WARNING''. Upgrade the warning to error in 9.0.

===== Backward Incompatible Changes =====
This proposal doesn't introduce new syntax, it only affects how some existing code could work by making the operator in some cases return true instead of false or throw warnings. However, since the affected use cases currently don't work (in a sense that they don't produce the result one would expect), existing code to be affected by this should be negligibly hard to come by.

===== Proposed PHP Version(s) =====
PHP 8.1.

===== RFC Impact =====
==== To SAPIs ====
None.

==== To Existing Extensions ====
Don't see a reason why they should.

==== To Opcache ====
None.

===== Open Issues =====
Make sure there are no open issues when the vote starts!

===== Unaffected PHP Functionality =====
Anything not related to ''instanceof''.

===== Future Scope =====
I'm currently pondering about extending type casts which would also improve type system and make syntax more consistent, but it's currently brewing in my head and I'm not going to let it out just yet.

===== Proposed Voting Choices =====
Accept this RFC (y/n, 2/3 majority required)?

===== Patches and Tests =====
* WIP patch: https://github.com/php/php-src/pull/6694

===== Implementation =====
After the project is implemented, this section should contain 
  - the version(s) it was merged into
  - a link to the git commit(s)
  - a link to the PHP manual entry for the feature
  - a link to the language specification section (if any)

===== References =====
Links to external references, discussions or RFCs

===== Rejected Features =====
Keep this updated with features that were discussed on the mail lists.