Fix unbound variable error when matching on const record subjects #5302
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IgorCastejon
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Jan 19, 2026
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| ValueConstructorVariant::ModuleConstant { .. } => { | ||
| // If the variable is a constant we shadow it with the refined type. | ||
| // We cannot convert it to a local variable as that would cause the | ||
| // code generator to look for a runtime variable that doesn't exist. | ||
| let mut refined_constant = variable.clone(); | ||
| refined_constant.type_ = type_; | ||
| let _ = self.environment.scope.insert(name, refined_constant); | ||
| } |
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I added this specific block to allow field access of const records (like user.name) to work inside the branch, matching the current behavior for local variables.
However, I noticed that Gleam doesn't do this type refinement for constants globally:
Gleam playground
Since the fix for the runtime crash doesn't strictly require this part (we could just ignore ModuleConstant entirely here), I have two questions:
- Should I keep or remove this code?
- Should I open a separate issue about global type refinement for constants (or is the current behavior deliberate)?
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| source: compiler-core/src/javascript/tests/case.rs | ||
| expression: "\npub type Wibble {\n Wibble\n Wobble(int: Int)\n}\n\nconst wibble = Wibble\n\npub fn main() {\n echo wibble\n // This 'wibble' shadows the const\n let wibble = Wobble(42)\n case wibble {\n // This matches the local variable, not the const\n Wobble(_) -> wibble\n }\n}\n" | ||
| --- | ||
| ----- SOURCE CODE | ||
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| pub type Wibble { | ||
| Wibble | ||
| Wobble(int: Int) | ||
| } | ||
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| const wibble = Wibble | ||
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| pub fn main() { | ||
| echo wibble | ||
| // This 'wibble' shadows the const | ||
| let wibble = Wobble(42) | ||
| case wibble { | ||
| // This matches the local variable, not the const | ||
| Wobble(_) -> wibble | ||
| } | ||
| } |
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Just something of interest I noticed while testing: if a const is defined but unused, it is properly optimized out of the generated JavaScript, but it still seems to have its name reserved in the variable generator.
Gleam playground
IgorCastejon
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Closes #5261.