GADT distinguished by boolean property not narrowed on test for truthiness

[svieira]

I have a Success / Failure tagged union type called Result, where the tag property is of the one of the two literal types in boolean. Success uses the literal type true and Failure uses the literal type false. I would expect that a test like if (someResult.success) would narrow the type in the else branch to Failure, but it does not.

TypeScript Version: 2.1.4-insiders.20161201 (on the Playground)

Code

type Success<T> = { success: true, result: T };
type Failure = {success: false, err: Error};
type Result<T> = Success<T> | Failure;

// Also possibly a bug - type assertion is necessary too
// Otherwise r is upcast to a Success<T> in spite of the type ascription
const r: Result<number> = { success: true, result: 1 } as Result<number>;

if (r.success) {
  r.result;
} else {
  r.err
}

Expected behavior:

No compilation error

Actual behavior:
Compilation error at the r.err:

Property 'err' does not exist on type 'Result'.
Property 'err' does not exist on type 'Success'.

[ahejlsberg]

Duplicate of #10564. It works as expected in --strictNullChecks mode.

The behavior without the as assertion is expected because control flow analysis has narrowed the actual type of r to Success<number>.

[svieira]

Fantastic - thanks for the explanation!