• Laravel Version: 5.5.42
• PHP Version: 7.3.0beta1
• Database Driver & Version: SQLite 3.24.0-1

Description:

Running a whereHas method off a hasMany relation blows up with:
ErrorException: compact(): Undefined variable: operator in file /var/www/testproject/vendor/laravel/framework/src/Illuminate/Database/Query/Builder.php on line 1243

Steps To Reproduce:

0. Define a hasMany relation on a model. Ensure that the child model on the relation has at least one relation other than to the parent.
1. Query the parent's hasMany relation - here's how I tripped over it:

$result = $parentModel->manyRelation()->whereHas('childOtherRelation', function ($query) use ($value) {
    $query->where('name', '=', $value);
})->firstOrFail();

2. Execute that query under PHP 7.3. I tripped over this by running a test case.
3. Bombs out with above error.

Looking at the chunk of code (addWhereExistsQuery in Query/Builder.php), I'm not seeing where $operator ever got defined.