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fix: prevent Any type from shadowing type errors in match arms #761

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fix: prevent Any type from shadowing type errors in match arms #761

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9 changes: 9 additions & 0 deletions src/fun/check/type_check.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -593,6 +593,15 @@ fn infer_match_cases(
let (s_rest, t_rest) = infer_match_cases(env.subst(&s), book, types, adt, rest, adt_s, var_gen)?;
let (t_final, s_final) = unify_term(&t1.subst(&s), &t_rest, bod)?;

// If the final type is `Any` but t1 (from non-unreachable arm) has type variables,
// we need to use t1's type to preserve those constraints
if let Type::Any = &t_final {
let t1_vars = t1.free_type_vars();
if !t1_vars.is_empty() {
return Ok((s_final.compose(s_rest).compose(s.clone()), t1.subst(&s)));
}
}

Ok((s_final.compose(s_rest).compose(s), t_final))
} else {
Ok((Subst::default(), var_gen.fresh()))
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