[5.6] Ignore non-where bindings in nested where() constraints

[staudenmeir]

Nested where() constraints merge all bindings from the given query, even the ones from other query parts.

This breaks queries that reference a relationship with WHERE bindings (e.g. MorphMany) in $withCount and use a nested where() constraint:

class ParentModel extends Model
{
    protected $table = 'parent';

    protected $withCount = ['children'];

    public function children()
    {
        return $this->morphMany(ChildModel::class, 'morphable');
    }
}

class ChildModel extends Model
{
    protected $table = 'child';
}

$query = ParentModel::where(function($q) {
    $q->where(1, 1);
});

The query doesn't work as expected because an extra binding is added:

dd($query->getBindings());
// expected: ['App\ParentModel', 1];
// actual: ['App\ParentModel', 'App\ParentModel', 1]

As a result 'App\ParentModel' is used for the second placeholder and 1 is ignored.

This is caused by Model::newQueryWithoutScopes() adding $withCount to every query.
So the query received by Builder::where() contains the $withCount bindings.

This PR solves the problem by ignoring non-where bindings when the nested query is merged.

Fixes #23957.

[taylorotwell]

Is it possible the nested where could legitimately have other bindings though?

[staudenmeir]

I tried to find a case, but couldn't think of one.

We could also solve this problem at the root with something like Model::newQueryWithoutScopesWithoutCount() (preferably with a better name).

This method would create a query without $withCount & $with and could also be used for INSERT/UPDATE/DELETE queries.

[mpyw]

I wrote a trait patch for this issue until this PR merged.

https://gist.github.com/mpyw/cb43c4cbd48d1ae1f63df949b1cc896b

[taylorotwell]

I kinda prefer the newQueryWithoutScopesWithoutCount approach if you have any better names ideas. Would be curious to see what that looks like.