zig/test/cases/bugs/828.zig

const CountBy = struct {
    a: usize,

    const One = CountBy {
        .a = 1,
    };

    pub fn counter(self: &const CountBy) Counter {
        return Counter {
            .i = 0,
        };
    }
};

const Counter = struct {
    i: usize,

    pub fn count(self: &Counter) bool {
        self.i += 1;
        return self.i <= 10;
    }
};

fn constCount(comptime cb: &const CountBy, comptime unused: u32) void {
    comptime {
        var cnt = cb.counter();
        if (cnt.i != 0) @compileError("Counter instance reused!");
        while (cnt.count()) {}
    }
}

test "comptime struct return should not return the same instance" {
    //the first parameter must be passed by reference to trigger the bug
    //a second parameter is required to trigger the bug
    const ValA = constCount(&CountBy.One, 12);
    const ValB = constCount(&CountBy.One, 15);
}