assertSessionHasErrors doesn't work with named MessageBag (only with "default")

[pl78]

• Laravel Version: 5.4.21
• PHP Version: 7.1.1

Description:

When doing test using assertSessionHasErrors method the test will fail if you validate the request using MessageBag with your own name (not "default") even thought the validation rules are set and validation works in browser.

Steps To Reproduce:

In my controller in store method I have:

$this->validateWithBag('answer', request(), [
    'body' => 'required'
]);

When I do assert in my test file like this:

$response->assertSessionHasErrors('body');

it fails with message "Session missing error: body".

It pass when in controller I change to:

$this->validate(request(), [
    'body' => 'required'
]);

I am using named MessageBag as I have two forms on one page with textarea field (both called "body").

[pl78]

As for now I've solved this issue with the below solution but propably there is a better way.

In Illuminate\Foundation\Testing\TestResponse.php I changed the line 544 (in assertSessionHasErrors method) from:

PHPUnit::assertTrue($errors->has($value), "Session missing error: $value");

to:

PHPUnit::assertTrue(collect($errors->getBags())->first()->has($value), "Session missing error: $value");

[themsaid]

Added this as a feature in a PR for 5.5: #19172