gleam-lang/gleam

Duplicate let when a variable is rebound after a directly matching case

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#5748 opened on May 24, 2026

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help wanted

Description

  • gleam 1.17.0-rc1
  • target javascript

A variable shadowed inside a directly matching case branch and then rebound after the case compiles to two let declarations with the same name, which is a SyntaxError.

pub fn go() -> Int {
  let n = 1
  case True {
    True -> {
      let n = 99
      n
    }
    False -> 0
  }
  let n = n + 5
  n
}

Compiles to:

export function go() {
  let n = 1;
  let $ = true;
  let n$1 = 99;
  n$1
  let n$1 = n + 5;
  return n$1;
}

SyntaxError: Identifier 'n$1' has already been declared.

This is the same duplicate let family as #3379 and #5612, which were fixed for pattern bound variables. It still happens when the inner binding is a plain let inside the branch block. I have a fix that tracks the names already declared in a function so a leaked branch declaration isn't reused, which also lets the #5493 same-name workaround go. Happy to open a PR if that approach sounds right.

Contributor guide

Tech stack
rust
Domain
tooling
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Familiarity with Gleam compiler source codeUnderstanding of JavaScript code generationKnowledge of previous similar issues (#3379, #5612)
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
35
Research direction
The issue is a duplicate let declaration bug in JavaScript output when a variable is rebound after a directly matching case. The author suggests tracking already declared names per function to avoid reusing leaked branch declarations. See previous fixes in #3379 and #5612. The fix should modify the JavaScript code generation module, likely in the compilation pipeline, to maintain a set of used variable names and ensure each new let uses a unique name.