Filter with "something is smallerType" don't work as negative about typescript HOT 5 OPEN

I certainly didn't mean that t => !isAnimal(t) should return t is Rock no matter what Something is, that would be bonkers. Yes, I'd expect Exclude<Something, Animal>, which in your example code was Rock.

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This is independent of filter():

function positive(t: Something) {
  return isAnimal(t)
}
// function positive(t: Something): t is Animal

function negative(t: Something) { 
  return !isAnimal(t)
}
// function negative(t: Something): boolean

I'm surprised that #57465 didn't do this. Inside negative() it certainly seems that !isAnimal(t) narrows in the way we expect:

function negative(t: Something) {
  if (!isAnimal(t)) {
    ((t));
    //^? (parameter) t: Rock
  } else {
    ((t));
    //^? ^?(parameter) t: Animal
  }
  return !isAnimal(t)
}

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Actually, it should be Exclude<Something, Animal> because in the case we add another one, we want all except the one we are filtering out.

https://www.typescriptlang.org/play/?ts=5.6.0-dev.20240624#code/C4TwDgpgBAggdgSwLYEMA2UC8UDeAoKKAIwCcIVgALALimBIFcIAaPAXwG489RIoAlAPYBjANZZcBYmQo0oAM3QBnFuy49w0GGgQQ4E-IVLkqtOAzRpWnbr2gBlQUghUEcAOYT4ydFAA+AiLiAdq6cOryDHDCwAiC+ghK3qhoABRKTi6Ubu60js6uHgCUtBkF2R5QibCIKZKEZMAMJPplWTkAdMayWJjY9ExcbNzC8UrAdBDjAIx5mYWe2IbSJnIDqsN4o3Djk+MATHPlOQZS3aYKyhvq27vAU8AAzEftlUtnMhfmltY3YxOuJQASSUAHlxNgANr3GbMPbAfZwmFPAC6HXkCDQ9xIqQmmAAfFUkrV0LiikUuAB6SmEWmEAB6AH4aj40JCUSN-nRssClAB1EjxRZQaEPaZIh6I+GPNEYrEQHF4wkAQkSyVJwHJVJpdIZzNS6owASEYn8sB0eiK7KgAFobYSlJRBBYACbEaCpE3Bc1hK0c7jUqAATSmcKBAHIkFAUAB3FBkKBAqBKBimihQVCiE4QAAeKCQYDQUw6QA

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@jcalz I don't think there's anything #57465 could have done to help here. We don't have negated types so you can't say t is not Animal--not even explicitly.

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But we don't need t is not Animal, only t is Rock.

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