Custom Product Attribute changes 'backend_type' when 'is_user_defined = 1' and get updated/saved in Admin Backend

[mhauri]

Preconditions

1. Magento: 2.1.4 to 2.1.6, maybe earlier versions are affected too.

Steps to reproduce

1. Create a custom Product attribute via Setup of a Custom Module:

[
    'required' => 0,
    'global' => ScopedAttributeInterface::SCOPE_GLOBAL,
    'group' => 'Test',
    'searchable' => 0,
    'comparable' => 0,
    'used_for_sort_by' => 0,
    'used_in_product_listing' => 0,
    'visible_in_advanced_search' => 0,
    'user_defined' => 1,
    'is_html_allowed_on_front' => 1,
    'visible_on_front' => 1,
    'input' => 'select',
    'source' => 'Test\Test\Model\Attribute\Source\Discount',
    'backend_type' => 'varchar',
    'label' => 'Discount',
    'position' => 1,
]

2. Go to the Backend Stores > Attributes > Product
3. Click on the newly created attribute and save it (with or without modifications).

Expected result

1. The backend_type doesn't change for the custom attribute.

Actual result

1. The backend_type got changed from varchar to int

[mhauri]

After digging into the code the problem starts in the file: module-catalog/Controller/Adminhtml/Product/Attribute/Save.php on line 199

if (is_null($model->getIsUserDefined()) || $model->getIsUserDefined() != 0) {
    $data['backend_type'] = $model->getBackendTypeByInput($data['frontend_input']);
}

So when the Attribute is user defined $model->getIsUserDefined() != 0 then the backend_type will be set based on the frontend_input $model->getBackendTypeByInput($data['frontend_input'])

The method getBackendTypeByInput() is defined in: module-eav/Model/Entity/Attribute.php

/**
     * Detect backend storage type using frontend input type
     *
     * @param string $type frontend_input field value
     * @return string backend_type field value
     * @SuppressWarnings(PHPMD.CyclomaticComplexity)
     */
    public function getBackendTypeByInput($type)
    {
        $field = null;
        switch ($type) {
            case 'text':
            case 'gallery':
            case 'media_image':
            case 'multiselect':
                $field = 'varchar';
                break;

            case 'image':
            case 'textarea':
                $field = 'text';
                break;

            case 'date':
                $field = 'datetime';
                break;

            case 'select':
            case 'boolean':
                $field = 'int';
                break;

            case 'price':
            case 'weight':
                $field = 'decimal';
                break;

            default:
                break;
        }

        return $field;
    }

By this method the frontend_input: select will always return int
As far as I can see there are core attributes which also have select as frontend_input and varchar as backend_type.

For the moment the solution will be to change the custom attribute to is_user_defined = 0 but I think this should be handled different for user defined attributes by not setting the backend_type based on the frontend_input

Or are there any reasons this is handled this way?