What should be correct autocomplete for username = email?

For a login form I think it's better to use autocomplete="username" for a username field, even if it is expected to be an email address.

I have no hard evidence for this but I expect this would be more friendly to password managers.

This article suggests the same.

A design document from Chromium aimed at developers also seems to suggest the same, and I quote from that:

<form id="login" action="login.php" method="post">
  <input id="emailfield" type="text" value="[email protected]" autocomplete="username">
  <input type="password" autocomplete="current-password">
  <input type="submit" value="Sign In!">
</form>

For a registration form, autocomplete="email" may make more sense.