The 1ML papers discuss this, ML implementations in reality are already incomplete with regard to Hindley-Milner, a quote:
"We show how Damas/Milner-style type inference can be integrated into such a language; it is incomplete, but only in ways that are already present in existing ML implementations."
"We show how Damas/Milner-style type inference can be integrated into such a language; it is incomplete, but only in ways that are already present in existing ML implementations."