function (Builder $q, string $morph) use ($value) {
    \PHPStan\dumpType($morph);
    $column = match ($morph) {
        //...

/** @return Builder<Opportunity> */
public function search(string $value): Builder
{
    return $this->builder->where(function (Builder $q) use ($value) {
        $q->whereHasMorph(
                'owner',
                [Customer::class, Lead::class],
                function (Builder $q, string $morph) use ($value) {
                    \PHPStan\dumpType($morph); // <--- *NEVER*
                    $column = match ($morph) {
                        Customer::class,
                        Lead::class => 'fullname',
                        default => null
                    };

                    if ($column !== null) {
                        $q->where($column, 'like', "%$value%");
                    }
                }
            );
    });
}

 /** @return MorphTo<EloquentModel, $this> */
public function owner(): MorphTo
{
    return $this->morphTo();
}