Error message for type guards are vague about rules

[DanielRosenwasser]

From #6538

interface Animal {
    animal;
}

interface Dog extends Animal {
    dog;
}

interface Car {
    car;
}

let thing: Dog | Car;

function isAnimal(x: Car | Dog): x is Animal {
    return "animal" in x;
}

Currently we just report

Type 'Animal' is not assignable to 'Car | Dog'.
    etc.

But that should be the elaboration. The first thing the user sees should be something like

A type guard's type must be assignable to its parameter's type.`
    Type 'Animal' is not assignable to 'Car | Dog'.
        etc.

[DanielRosenwasser]

This is a really easy bug to fix for anyone who wants to take a stab at this.

[Vinatorul]

I am trying to do this. It was not hard to add new diagnostic message, but I am not sure where should I check that I am checking the assignability of the guarded type. Is there a special way to find it out like backtrace?

[DanielRosenwasser]

checkTypePredicate is where the magic happens. Specifically, this has to do with the call to checkTypeAssignableTo. You'll probably want to use chainDiagnosticMessages to create a DiagnosticMessageChain.

[DanielRosenwasser]

Thanks @Vinatorul!