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Integer range patterns ending at 2147483647 generate incorrect JavaScript

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@fhammerschmidt is already working on this.

Since Oct 2, 2026.

  • #8717 by @fhammerschmidt — open

Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
70/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Active
Tech stack
javascript, ocaml
Domain
compilers

Research direction

Use the maintenance/12.x compiler with the supplied ReScript switch, first inspecting the generated JavaScript for the integer-range condition. Reproduce the boundary cases and compare the compiler's handling of the exclusive upper bound with the signed 32-bit limit. Done means the generated check accepts 2147483645–2147483647 and rejects 2147483644, including the other listed switch shapes.

Written by the indexing model from the issue text.

Description

An integer or-pattern ending at 2147483647 (the maximum signed 32-bit integer) compiles to an impossible bound check and rejects values that should match.

Reproduced with the ReScript 12.3.1 compiler on macOS arm64, using the maintenance/12.x code both before and after the change in #8714.

type response = {status: int}

let inRange = (x: response) =>
  switch x {
  | {status: 2147483645 | 2147483646 | 2147483647} => true
  | _ => false
  }

Generated JavaScript, identical before and after #8714:

function inRange(x) {
  let match = x.status;
  if (match >= 2147483645) {
    return match < -2147483648;
  } else {
    return false;
  }
}

Expected and observed results:

Input status Expected Actual
2147483644 false false
2147483645 true false
2147483646 true false
2147483647 true false

The generated condition requires the same integer to be at least 2147483645 and less than -2147483648, so it can never succeed. This appears to be overflow when converting the exclusive upper bound 2147483647 + 1 into a 32-bit integer literal.

The same failure also reproduces with a plain integer switch, nested records, tuples, variant payloads, arrays, and switches on function-call results.

This was found while reviewing #8714 and is a separate, pre-existing bug: the unsigned-range fix in that PR neither introduces nor fixes it.

Dominant language
OCaml
Stars
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Forks
484
Avg merge
19h 46m
Merged PRs (30d)
81

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