gleam-lang/gleam
Duplicate let when a variable is rebound after a directly matching case
Closed
#5,748 opened on May 24, 2026
help wanted
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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.