pub fn main() !void {
    var str: [4]u8 = undefined;
    var a = 5;
    str[1] = if (a > 5) 'a' - 10 else '0';
}

./test.zig:3:5: error: variable of type 'comptime_int' must be const or comptime
    var a = 5;
    ^
./test.zig:4:18: note: referenced here
    str[1] = if (a > 5) 'a' - 10 else '0';
                 ^
/usr/lib/zig/std/start.zig:334:40: note: referenced here
            const result = root.main() catch |err| {