TS 3.2.0-rc seems to forget union type refinements more eagerly

[Jessidhia]

TypeScript Version: 3.2.0-rc

Search Terms:
refinement

Code

(EDIT: incorrect reproduction, see correct example at #28568 (comment))

function test(x: { type: 'a'; body: string | null } | { type: 'b'; body: number | null }): string {
  if (x.body === null) {
    return 'null'
  }
  x.body.toString() // ok

  if (x.type === 'a') {
    return x.body.toString() // x.body may be null
  } else {
    return x.body.toString() // x.body may be null
  }
}

Expected behavior:
The refinement of x.body should persist (it does in 3.1)

Actual behavior:
TS forgets that it has already refined x.body to be not null.

[mmis1000]

Maybe it guess x.body.toString() may has side effect and fail the refinements?

[Jessidhia]

Oh, no, removing that line has no effect. I just added it to demonstrate that the initial refinement is working.

[ahejlsberg]

I can't reproduce a difference between 3.1 and 3.2. In --strictNullChecks mode the example errors on both, and without --strictNullChecks the example passes on both.

As an aside, the error in --strictNullChecks mode is due to a known design limitation in the control flow analyzer--upon encountering a discriminant check we narrow based on that check but forget about other narrowings that were already in effect.

[Jessidhia]

That is odd. This is an error I only encountered in existing code once I updated to TS 3.2, but my snippet indeed also displays the error in 3.1...

[Jessidhia]

Oh, I found the version that errors in 3.2 but not 3.1; both with --strict:

3.1:

3.2:

function test(x: { type: 'a'; body: string | null } | { type: 'b'; body: number | null }): string {
  if (x.body === null) {
    return 'null'
  }

  switch (x.type) {
    case 'a':
      return x.body.toString() // x.body may be null
    case 'b':
      return x.body.toString() // x.body may be null
  }
}

[ahejlsberg]

@Kovensky Yes, that is due to #27612. We weren't correctly resetting our narrowing assumptions in switch statements. It happened to work in some cases, including yours, but generally was wrong and inconsistent with corresponding if statements.

[typescript-bot]

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