compiler/docs/ARRAY_DEF.md

#[ArrayDef] compile-time array contracts

#[ArrayDef] attaches key/value type information to a property declared exactly as array. It supports both Zend classes and #[Native] classes and has no runtime metadata or per-read overhead.

class Index
{
    #[ArrayDef(Type::String)]
    public array $names = []; // list<string>

    #[ArrayDef(Type::Int, Type::String)]
    public array $labels = []; // map<int, string>
}

One argument defines a list value type. Two arguments define a map key type and value type. Map keys are restricted to Type::Int or Type::String. ClassName::class is therefore valid only as a list element type or as the second (value) argument of a map.

For direct writes whose expression types are known, the compiler either emits the normal write unchanged or reports a fatal type error. An any key/value is checked with PHPX exact-type helpers at runtime. No coercive intval() or string conversion is performed.

List writes support [], an existing integer index, and the exact append form $object->property[count($object->property)]. Existing-index writes use php::safeIndex(); negative and out-of-range indexes fail at runtime. Maps do not support [] append writes.

The contract intentionally applies only to direct element assignment lowered by TypePHP. Reads and in-place operators are unchanged. Values passed through dynamic functions, callbacks, Reflection, eval(), or other ZendVM escape paths are outside the contract and have undefined behavior from ArrayDef's perspective.