Type guard functions do not narrow within switch/case statements

[subvertallchris]

TypeScript Version: 2.4.2

Code

interface GeneralInterface {
  value: string;
  foo?: true;
  bar?: true;
}

interface FooInterface {
  value: 'FOO';
  foo: true;
}

function guard<T>(broadObject: any, comparison: string) : broadObject is T {
  return broadObject.value === comparison;
}

function passingTestIf() {
  const broadObject: GeneralInterface = { value: 'FOO', foo: true };
  if (guard<FooInterface>(broadObject, 'FOO')) {
    const narrowed = broadObject;
    return narrowed.value === 'FOO';
  }

  return false;
}

function failingTestCase() {
  const broadObject: GeneralInterface = { value: 'FOO', foo: true };
  switch (true) {
    case (guard<FooInterface>(broadObject, 'FOO')): {
      const notNarrowed = broadObject;
      return notNarrowed.value === 'FOO';
    }
  }

  return false;
}

Expected behavior:

My hope is that the behavior within the body of the case would match the body of if: the const currently defined as notNarrowed should be identified as type FooInterface.

Actual behavior:

Within the body of an if statement, the type guard is evaluated and successfully narrows the object.

When evaluated as part of switch/case, the const notNarrowed is still seen as a GeneralInterface, despite being proven to be the more arrow FooInterface.

Apologies if I am missing anything, I greatly appreciate your time and work!

[kujon]

See #8934

[subvertallchris]

Interesting and unfortunate, but thank you for linking me to that. I tried to find something like that but was unable. Is there any chance that this would be considered for resolution now? I'll leave this open for a maintainer to bring the Closehammer down. Thank you!

[RyanCavanaugh]

You can leave a comment in the linked issue. We periodically review them to see if things need to be revisited

[subvertallchris]

No problem, I'll close this and comment there. Thanks!