Are you seeing this on any released version, or only against master?

And are you sure *that exact code* reproduces the problem?

Description:
------------
If an object has a __toString() method define in a trait, the object could not be converted to string.

Test script:
---------------
<?php
declare(strict_types=1);

namespace Traits {
    trait ToString
    {
        abstract public function toString(): string;

        public function __toString(): string
        {
            return $this->toString();
        }
    }
}

namespace Test {

    use Traits\ToString;

    class MyStr {
        use ToString;

        public function toString(): string
        {
            return 'mystr';
        }
    }

    class Foo {
        use ToString;

        public function toString(): string
        {
            return 'foo';
        }
    }

    $foo = new Foo();
    echo $foo;
    echo PHP_EOL;
    $s = new MyStr();
    echo $s;
}

Expected result:
----------------
foo
mystr

Actual result:
--------------
foo 
Recoverable fatal error: Object of class Test\MyStr could not be converted to string

Exactly what version of PHP are you using?

The 7.3.0 include in XAMPP 7.3.0.

In 7.2.4 the problem not occure.

Duplicate of bug #77291 which was fixed in 7.3.1.