PHP Traits: define property and error "definition differs"

Question

I have the following code and it works correctly:

abstract class ParentClass
{
    public $flag = true;
}

class ChildClass extends ParentClass
{
    public $flag = false;
}

var_dump((new ChildClass())->flag); // false

But if I use Trait it leads to Fatal Error

ParentClass and TFlag define the same property ($flag) in the composition of ChildClass. However, the definition differs and is considered incompatible. Class was composed in...`

abstract class ParentClass
{
    public $flag = true;
}

trait TFlag
{
    public $flag = false;
}

class ChildClass extends ParentClass
{
    use TFlag;
}

var_dump((new ChildClass())->flag);

UPDATED:

It tested on PHP 7.1, 7.2, 7.4

Why is the following definition compatible for class inheritance and for logic, but not incompatible for trait?

• public $flag = true;
• public $flag = false;

UPDATED

I can override a method but not a property (it works):

abstract class ParentClass
{
    public function flag(): bool
    {
        return true;
    }
}

trait TFlag
{
    public function flag(): bool
    {
        return false;
    }
}

class ChildClass extends ParentClass
{
    use TFlag;
}

var_dump((new ChildClass())->flag()); // false

Answer 1

The PHP Documentation is explicit about this (examples 12 and 13).

https://www.php.net/manual/en/language.oop5.traits.php#language.oop5.traits.properties

If a trait defines a property then a class can not define a property with the same name unless it is compatible (same visibility and initial value), otherwise a fatal error is issued.