extern fn ext() usize;
var bytes: [ext()]u8 = undefined;
export fn f() void {
    for (&bytes, 0..) |*b, i| {
        b.* = @as(u8, i);
    }
}

// error
//
// :2:16: error: comptime call of extern function
// :2:12: note: types must be comptime-known
