Flag 'instanceof' expressions that are provably always true or false

Suggestion

When refactoring code, TypeScript is generally very good at finding and reporting situations where existing code will break due to type changes. This allows "refactoring with confidence". This isn't surprising given that one of the primary goals of TypeScript is to "statically identify constructs that are likely to be errors".

There's one common case where TypeScript is "blind" and doesn't report errors when types are refactored. This case involves the use of the 'instanceof' operator. This operator can be used in situations where it makes no sense -- e.g. where the type specified on the LHS has no possible relation to the type specified on the RHS.

To help developers avoid this common programming error, I propose that TypeScript report an error for any 'instanceof' expression that is provably always true or false at compilation time. Any such expression will likely be unintended by the programmer and should be flagged as errors. At best, such operations represent unnecessary code that imposes runtime overhead.

Example

// ClassB is a subclass of ClassA.
class ClassA {}
class ClassB extends ClassA {}

// ClassC has nothing to do with either
// ClassA or ClassB.
class ClassC {
    method1() {}
}

function function1(param1: ClassB) {
    // Static analysis can prove this expression
    // will always evaluate to false. It's a common
    // source of programming errors, especially during
    // refactoring.
    if (param1 instanceof ClassC) {
        // This call isn't even valid given that param1
        // was declared as type ClassB.
        param1.method1();
    }

    // Static analysis can prove this expression will
    // always evaluate to true, so at best it's unnecessary
    // runtime overhead and more commonly is a behavior
    // that is unintended by the programmer.
    if (param1 instanceof ClassA) {
        // ...
    }
}

Are there any legitimate uses of 'instanceof' where the result is statically provable to be true or false in all cases? I can't think of any, but it's possible I'm overlooking some specialized cases. If there are, this check could be added as an optional compiler switch to preserve the current behavior.

Inizia esaminando gli esempi ClassA, ClassB e ClassC dell’issue e il comportamento esistente del controllo dei tipi con instanceof nel compilatore TypeScript. Determina come devono essere diagnosticati i casi sempre veri e sempre falsi, incluse eventuali eccezioni legittime. Il lavoro è completato quando il compilatore segnala i casi previsti, gli esempi sono coperti e viene preservato l’uso valido di instanceof.