ts-expect-error before assignment from for expression

[bbrk24]

In this code, it's impossible to place the @ts-expect-error directive correctly:

declare let t: [number, number, number]

// @ts-expect-error I can't prove this has three elements but it does
t = for i of [1..3]
  Math.sqrt i

The current spelling complains about the type mismatch and about an unused expect-error directive, as it compiles to

declare let t: [number, number, number]

// @ts-expect-error I can't prove this has three elements but it does
const results=[];for (let i1 = 1; i1 <= 3; ++i1) {
  const i = i1;
  results.push(Math.sqrt(i))
};t = results

Current workaround:

t = (for i of [1..3]
  Math.sqrt i
) as any

[edemaine]

This seems like the same issue as #1113. Could you move your example to there?

[bbrk24]

I think it's different. That issue can plausibly be fixed by removing a newline from the generated code, and emitting for (const key in obj){const value = obj[key];. This issue requires transposing the comment across the entire body of the for loop, which may be arbitrarily long and isn't necessarily collapsible (e.g. if it contains line comments or a multi-line template string).

[edemaine]

Ah, right, sorry.

[edemaine]

Do you have a proposal for how to fix this? I don't think we can use t in place of results here because t might change in the loop and then semantics change.

One better workaround I think:

sqrts := for i of [1..3]
  Math.sqrt i
// @ts-expect-error I can't prove this has three elements but it does
t = sqrts

It would be nice if this worked, but |> currently can't follow a loop like this:

t = for i of [1..3]
  Math.sqrt i
|> as [number, number, number]

Or perhaps this would be even cleaner, but also doesn't parse as intended:

t = for i of [1..3]
  Math.sqrt i
as [number, number, number]

Back to comments, one thing I would find somewhat natural to write is

t = // @ts-expect-error
  for i of [1..3]
    Math.sqrt i

Unfortunately @ts-expect-error must come on the line before, which seems hard to guarantee...