pub fn testFn(str: []const u8) u8 {
    return str[0] - 'a';
}

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

./test.zig:7:59: error: cannot store runtime value in type 'comptime_int'
    str[1] = testFn("abcd") + if (testFn("abcd") > 5) 'a' - 10 else '0';
                                                          ^
/usr/lib/zig/std/start.zig:334:40: note: referenced here
            const result = root.main() catch |err| {
                                       ^