[Downgrade PHP 7.4] Contravariant arguments

[leoloso]

Feature Request

Downgrade new feature from PHP 7.4 to its equivalent PHP 7.3 code

Diff

class ParentType {}
class ChildType extends ParentType {}

class A
{
    public function contraVariantArguments(ChildType $type)
    { /* … */ }
}

class B extends A
{
-    public function contraVariantArguments(ParentType $type)
+    /**
+     * @param ParentType $type
+     */
+    public function contraVariantArguments($type)
    { /* … */ }
}

[leoloso]

To downgrade it, we only need to replicate the same argument types from the parent, without double-checking if these are contravariant (i.e. if the type in the child class is a parent to the type in the parent class). That's because it must be contravariant, or otherwise it will already fail for PHP 7.4.

The rule will be similar to MakeInheritedMethodVisibilitySameAsParentRector

[leoloso]

Actually, this is more complicated than I mentioned in my previous comment: the change would need to be done on the parent class, not on the child class!

That's because the child class will have a more generic argument type, so it can't be reduced to the more specific one from the parent class, or invocations passing that more generic type will fail.

A possible solution is to completely remove the parameter type from the child class, which is called Parameter type widening. However this works starting from PHP 7.2 only.

To make it work with PHP 7.1 and below, we can directly remove the parameter types on both the parent and child methods, replacing them with phpdocs.

[leoloso]

Should make sure if there is a proper solution for #4754, and implement it, before tackling this issue

[leoloso]

Solution: remove the type completely, switching from covariant argument type, to parameter type widening. Then, this latter feature can be downgraded for PHP 7.2 through new rule DowngradeParameterTypeWidening