Why is this needed?

Let me try to explain:

The test case for the assertResponseHeaderNotSame

public function testAssertResponseHeaderNotSame()
{
    $this->getResponseTester(new Response())->assertResponseHeaderNotSame('Cache-Control', 'public');
    $this->expectException(AssertionFailedError::class);
    $this->expectExceptionMessage('Failed asserting that the Response does not have header "Cache-Control" with value "no-cache, private", value of header "Cache-Control" is "no-cache, private".');
    $this->getResponseTester(new Response())->assertResponseHeaderNotSame('Cache-Control', 'no-cache, private');
}

failed with:

Failed asserting that exception message

'Failed asserting that the Response does not have header "Cache-Control" with value "no-cache, private", value of header "Cache-Control" is not "no-cache, private".'

contains

'Failed asserting that the Response does not have header "Cache-Control" with value "no-cache, private", value of header "Cache-Control" is "no-cache, private".'.

• Please notice the not in the actual exception message.

This is happening because the assertResponseHeaderNotSame assertion uses a ResponseHeaderSame constraint internally. This constraint is wrapped by a LogicalNot which is in fact the reason all messages for this constraint are negated.

So the best idea I had so far is to "ignore" the logical not case.