Bug: Changing an immutable parameter

[alexoransky]

Odin release dev-2026-06:285f6d87b

Consider the following code:

package main

import "core:fmt"

PersonData :: struct {
    health: int,
    age: int
}

MyUnion :: union {
    f32,
    int,
    PersonData
}

print_union_data :: proc(data: MyUnion) {
    switch &v in data {
    case int:
        v = 10
    case f32:
        fmt.println("f32")
    case PersonData:
        fmt.println("PersonData")
        fmt.println(v)
    }
}

main :: proc() {
    val: MyUnion = int(12)
    fmt.printfln("Before the call: %v", val)
    print_union_data(val)
    fmt.printfln("After the call: %v", val)
}

Run:
cd
odin run .

The expected output is

Before the call: 12
After the call: 12

because the data parameter in the print_union_data() is passed by value.

Instead we are getting:

Before the call: 12
After the call: 10

Curiously enough, the issue disappears when the PersonData type is removed from MyUnion.

[cy5d]

Hey! This is expected behavior. Odin passes large values by pointer rather than performing a deep copy, primarily for performance reasons.

See: #2971

[alexoransky]

Hello!

You are mixing implementation details that are based on some stack usage considerations with the expected behavior that is based on the procedure signature.

To me, this warrants a compilation error or at least a warning for this line:
switch &v in data {