Custom type guard in getters is broken

[tinganho]

TypeScript Version:

nightly

Code

class A {
    get a(): this is A {
        return true
    }
}

Expected behavior:

The getter type guard should not complain

Actual behavior:

tsc
Library/DataStore.ts(55,27): error TS1228: A type predicate is only allowed in return type position for functions and methods.
Library/DataStore.ts(103,20): error TS1228: A type predicate is only allowed in return type position for functions and methods.

[RyanCavanaugh]

Is this a regression?

[mhegazy]

this is by design. typeguards only exist on signatures, and not properties. getters/setters are properties, and that caused different issues. this is a recent change, so if used nightlies before, then it would have worked. see #7067 for more details.

[tetsuharuohzeki]

this is by design. typeguards only exist on signatures, and not properties. getters/setters are properties, and that caused different issues.

I seem property cannot be a typeguard would be a problem in some cases of an union type (e.g. Option<T> type). They make the property which checks the current type a constant value instead of calling a method to optimize the performance.

code example:

type Option<T> = Some<T> | None<T>;

class Some<T> {
  constructor(private v: T) {
    this.isSome = true;
  }

  isSome: this is Some<T>;
}

class None<T> {
  constructor() {
    this.isSome = false;
  }

  isSome: this is Some<T>;
}

// ---------------------------

if (option.isSome) {
  // some
}
else {
  // none
}