Description

Each is branch inside a when block shares the same scope. Declaring a variable in one is branch prevents declaring the same variable name in another branch, which is incorrect — each branch is mutually exclusive and should have its own scope, like if/otherwise blocks do.

Repro

do classify(x int) -> (label string) {
    when x {
        is 1 {
            mut label string = "one"
            return label
        }
        is 2 {
            mut label string = "two"
            return label
        }
        default {
            mut label string = "other"
            return label
        }
    }
}

do main() {
    println(classify(1))
    println(classify(2))
    println(classify(99))
}

Expected: Compiles and prints one, two, other. Each is block has its own scope.

Actual:

error[E4003]: variable 'label' already declared in this scope (line 4)
  --> test.ez:8:13

Where to look

ezc/src/typechecker/typechecker.c — the NODE_WHEN_STMT case. Each is branch body needs scope_create()/restore around it, same as if/otherwise branches already have. The default branch needs the same treatment.